How can we trust someone and accept him as Guru? Can we know or assess the spiritual level of a person? Are there any references for choosing one?
What should we do if we realize that the guru we chose wasn't the right one in terms of knowledge?
As per the scriptures, a right guru must be fulfilling two important qualities:
So the scriptures say as below:
tad vijñānārthaṃ sa gurum evābhigacchet samit-pāṇiḥ śrotriyam brahma-niṣṭham [Mund. Up. - 1.2.12]
-To know That (God) he (the seeker) goes to a guru who is well versed in scriptures and situated in Brahman (God realized) .
tasmād guruṃ prapadyeta jijñāsuḥ śreya uttamam
śābde pare ca niṣṇātaṃ brahmaṇyupaśamāśrayam [SB - 11.3.21]
Therefore, the seeker approaches a good and noble guru who is skillful in the knowledge of the scriptures and the supreme and who having taken the shelter of Brahma stays satisfied in Him.
After following a guru for few years if you find you are not improving or he is not the right person, then the only thing you should do is to stop following him and without any ill feelings towards him, just try to find someone else.
Technically one can do a lot of things after he finds himself cheated or a prey to the media hype, but he should focus upon continuing his development rather than getting revenge minded and focusing on someone else's ruin. Those negative feelings will only degrade him further instead of helping him move up.
Finding guru is actually a subjective thing. Even while one guru may work for a seeker, he may not be good for another one. And again, how would you know if he is God realized? Any one may show some cheap miracle or some spiritual power and can act as a guru. So it is generally said that it is not the student who finds the guru, it is the guru who finds the student. For example, Vivekananda first didn't accept Ramakrishna as his guru, even he mocked him. But later upon more interaction he felt his divinity and accepted as guru. So without properly analyzing it is very hard to find out who is a true guru and who is not. It is because true saints even act opposite to their nature sometimes. So from my experience I would say if one has faith in God and genuine interest, then eventually he will find the right guru even if he falls prey to others many times.
Adding more info regarding changing guru upon pbvamsi's suggestion. The thing about changing guru is that, it should be done only when its necessary and required. So I said one can change guru when he is not finding any improvement after following him.
Because not all gurus are perfect or of the same caliber and qualification, one may not find perfection by following only one guru. So the scripture says knowledge cannot be stable through one guru alone:
na hyekasmādgurorjñānaṃ susthiraṃ syātsupuṣkalam [SB -11.9.31]
- Not through one guru knowledge becomes steady and complete.
There are also real life examples when a guru sends his disciple to someone else for further improvements. Also the other thing is that, one guru may be perfect in yoga but doesn't know about devotion. So a student have to change his guru if he wants to learn devotion, knowledge or something else like it. So there is provision for changing guru. But only when it is necessary and required. If one finds a guru who is perfect and satisfies the above mentioned two conditions, there is no reason for someone to change guru. Moreover, in that case changing guru will break ananyata (complete surrender to only one) and the student cannot proceed further.
So your saying that a guru cannot be changed is right and true, but only when one has accepted the guru as one's true master and the guru also has accepted him as his dear disciple. In those cases, the guru and disciple become one soul in two bodies. Guru and shisya relationship is the most noble, divine and pure of all. It is one bond that never breaks. So before one has found his true guru, he can change many others. But once he has found him, there is no change.
Adding few points to jabahar's answer:
There is no privilege to change Guru in the disciple's lifetime. One has to take enough time in choosing a right Guru. Infact it is not the student who finds the guru, it is the guru who finds the student. Based on one's strong determination, he may get the right Guru.
Few references to add are:
(1) Once a disciple came to Narasimha Saraswathi and begged Him to accept him as disciple showing the anger of his current Guru as the reason, but Narasimha Sarawathi explained what kind of Guru one will get and sent him back to his Guru.
(2) Once Indra left Bruhaspathi and approached Viswarupa to be as his Guru. Later Indra approached Dattatreya and pleased Him to be his Guru! Dattatreya explained the Guru principle and denied to accept Indra as His disciple. He said there is no chance to change Guru and taught him to approach Bruhaspathi again.
So before choosing a person as Guru one can test Him multiple times but after choosing Him as Guru there is no chance to change! Even if disciple stops following, He will uplift disciple's life, He will follow disciple in all the lifes to come(reference is Bhadrasheela's story in Datta Purana). But if one denies his own Guru then no one can save him.
After one makes stong determination, he will be privileged to meet many sadhus. With whom one's heart becomes peaceful and experiences sat-chit-ananda (true bliss eternal), in whose presence one gets courage and will power to solve all problems(a feeling that they will get solved by themselves), in whose presence all the worries & sorrows are forgotten, above all in whose presence one feels
he is my savior, my relative, such a person can be approached as Guru.
I am only adding to Be Happy's answer. I am in agreement with most of his answer. My points are in addition to his. First, the third qualification of the guru is sinlessness. The guru should be above any moral approach. This includes brahmachari.
Second, one cannot change gurus. As Ramakrishna Paramahamsa says in the The Gospel of Sri Ramakrishna (Appendix A: Keshab at Dakshineswar, available here http://www.belurmath.org/gospel/index.htm) -
[Sri Ramakrishna] "One must not look on one's guru as a mere human being: it is Satchidananda Himself who appears as the guru. When the disciple has the vision of the Ishta, through the guru's grace, he finds the guru merging in Him.
"What can one not achieve through simple faith! Once there was an annaprasana ceremony in a guru's house. His disciples volunteered, according to their powers, to supply the different articles of food. He had one disciple, a very poor widow, who owned a cow. She milked it and brought the guru a jar of milk. He had thought she would take charge of all the milk and curd for the festival. Angry at her poor offering, he threw the milk away and said to her, 'Go and drown yourself.' The widow accepted this as his command and went to the river to drown herself. But God was pleased with her guileless faith and, appearing before her, said: 'Take this pot of curd. You will never be able to empty it. The more curd you pour out, the more will come from the pot. This will satisfy your teacher.' The guru was speechless with amazement when the pot was given to him. After hearing from the widow the story of the pot, he went to the river, saying to her, 'I shall drown myself if you cannot show God to me.' God appeared then and there, but the guru could not see Him. Addressing God, the widow said, 'If my teacher gives up his body because Thou dost not reveal Thyself to him, then I too shall die.' So God appeared to the guru-but only once.
"Now you see, because of faith in her guru the disciple herself had the vision of God and also showed Him to her teacher. Therefore I say, 'Even though my guru frequents a grog-shop, still to me he is the embodiment of Eternal Bliss.'
"All want to be the guru, but very few indeed want to be the disciple. But you know that rain-water doesn't collect on a high mound; it collects in low land, in a hollow.
So the question becomes what if one has determined he has taken a guru with a low character? One should then worship the guru from a distance. To do worship does not require interaction or to be in the presence of the person. One can go and mingle with others who one may determine are on a more sattwic path. If, after doing such for a while, one still wants to take another guru, then the test of the guru should be done rigorously and the fact of having already take another as a guru should be explained to the new guru upfront for his advice.
First understand the meaning of the word Guru in Sanskrit tradition, Guru has to be a realized being. It's not possible for ordinary being to know if a person is realized or not.
So the safest bet is to know how integrated the person is to Shastras and their Guru parampara. Although, there are many great exceptions like Sri Ramana Maharshi who don't have a Guru parampara and don't have a successor.
A Guru has to evaluated on the 4 parameters:
ShastraPramana: The original scriptures of the tradition of the Guru parampara. Are the teachings and claims made by Guru backed by Shastras ? How integrated the Guru is to those Shastras ? Are the disciples encourages to read and follow those original scriptures ?
Apta Pramana: If the Shastras work then there must be other Gurus i.e. realized masters in the present and past who would have "realized" the experience mentioned in Shastras and would have written commentaries based on their self realization. *(Think of them like experiments that have been replicated by other "experts" based on the original publication (shastras))*
Atma Pramana: The self experience of the Guru itself. Are they in sync with the Apta Pramana and Shastra Pramana i.e. Is Guru able to replicate the results specified by Shastras and do they correlate the results with other publications ?
Sakshi Pramana: The experience of disciple itself. Can the experience of disciple itself conform to the results recorded as the result of ShastraPramana, AptaPramana and AtmaPramana.
This system is a very vigorous system of evaluation. It is often very very difficult to find a Guru who passes the 4 parameters listed above. But if you find one who passes on all counts immediately go to him/her.
Till one finds a such a Guru its best to follow an Acharya who is integrated to Shastras. Every Kriya suggested by the Acharya has to be backed up the "Shastra" and insist on producing the original Sanskrit verse.
For eg, if one has to bend the body in a certain way for Yoga it has to be based on the original Sanskrit verse. If one has to put certain material into Homa, it has to be specified in the Sanskrit verse.
Thankfully, there are many traditional Acharyas who follow the Shastras and more then glad to produce the original Shastras.
Even among Gurus, there are different Gurus who can "transmit" different areas of Shastras. When the Guru thinks the disciple is ready and has nothing more to add then Guru send the disciple to the "next" Guru. This process goes on till disciple is sent to the Sadguru. (Sadguru, as per Shastras, its a very specific Sanskrit term)
Sastra Praman, Apta Pramana and Atma Pramana: A Dialogue between Swami Nithyananda and Rajiv Malhotra http://hinduismnow.org/blog/2017/02/26/sastra-praman-apta-pramana-atma-pramana-dialogue-swami-nithyananda-rajiv-malhotra/
"A Discussion with Nithyananda: on God vs. Sadashiva, Why Wear Gold, Attacks against Hinduism & More" https://www.youtube.com/watch?v=SNAHZpRl3go ... original video of the transcript in 2
Where will you search and how will you know who is satguru and who is not satguru? You have no way to judge, isn't it? So, "how do I seek something?" You just seek. You simply seek, "I want to know." The more you become, "I do not know," the deeper your seeking is, isn't it? Seeking does not mean seeking something. Seeking means, seeking that which you do not know. If you have to seek, you should not make any assumptions, isn't it so? …
If a vacuum of "I do not know" happens within you, satguru will happen to you. You don't have to search, because you don't know how to search. If you search with your intellect, naturally you will search for that kind of person with whom you are most comfortable, isn't it?
See, if you seek a friend, what kind of friend do you seek? Do you seek a friend who punctures your ego every day? You seek a friend who nourishes you ego every day, isn't it? Yes or no? If somebody punctures your ego, he becomes your enemy, isn't it? **So, even if you seek a guru, you will only seek with this context—and that kind of guru, who comforts you, is no use to you. …
(However,) if you are seeking a guru for liberation, then, if you sit with him, you feel threatened. You want to run away from him, but at the same time you don't want to leave him for a moment.** If you constantly feel threatened by him and at the same time you want to be with him, he is a good guru for you. If you are very comfortable with him, he's not a good guru for you. You must be constantly uncomfortable with him. At the same time, you long to be with him. That is a good prescription for you.
—Sadhguru Jaggi Vasudev, via YouTube, emphasis added