It is okay for everyone to follow the Guru of their choice, complete 100% trust in the Guru is necessary, but what if the Guru himself is not in the right direction? so it is very important to confirm someone's credentials before accepting the Guru if you are serious about the subject.
To learn Physics, would you read a text-book of Physics printed by someone privately, or read a book that refers to original scientific papers and publications? You would beleive E-mc^2 is true only if you could trace to Einstein's original work.
One way would be the lineage, one cannot be a Guru by acquiring great power and declaring hiself as God 'unilaterally', fooling gullible people whose only basis is personal faith.
Take an example of Prabhu-. and Sai-baba. (I am not a follower of Prabhu-. myself but am interested in Vaishnana philosophies and respect his achievements in spreading Vaishnavism)
-Who is the Guru's Guru?
PP-allegiance line can be traced to Chaitanya and also MadhvaCharya, both who are well known, I need to say no further word about them.
SB-?
-What is the basis of Guru's faith?
PP-A BrahmaSutra Bhashya in the Gaudiya line, also based on Dvaita which is undefeated since the inception and survived debates across centuries.
SB-? not sure of the basis, definitely not related to original work on Vedas/Vedanta
-What is the connection with the root of the faith?
PP-Based on Vedanta which is the source (understanding of Gita can be traced to Vedanta)
SB-Borrowed from someone without proper acknowledgement
-Notoriety if any.
PP-I have never heard any negative comment on his character, only some criticism on Philosophical basis.
SB-I need not say, just search for Sai Baba in Google.
In India you will find thousands of Gurus masquerading as Godmen. Anyone can read a book on Gita and preach. Just perform some staged miracles and get powerful contacts (such as political), you will get hundreds of followers who are depressed and you can feed on their hope for years.
These Gurus get easy publicity as the real Gurus have no wish to build legions of followers but are occupuied more with practising what they preach and preserving, spreading their learnings rather than have followers who follow blind-faith.
These self-procalimed Gurus also keep away from debates on the real subject as they themselves have no basis and vaguely say things based on work of others, adding their own nice words (everyone is good, great, should be happy, this and that).
Most of all, I am surprised to find so many followers of Sai Baba among educated caste-brahmins who could have had more faith on their own Gurus who can trace their line to Sankara, ramanuja, Madhva,...
I don't understand what people find in him, it is a great mystery to me, maybe he hypnotises. As for the usual words that he build hospitals, schools etc.,, I could have done the same if not better if I had money and followers like him, what's the big deal??????? Philanthrophists have done many times better than him, they do not publicize.