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:
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:
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
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) -
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.