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 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 desciple 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!