There are some primary yard sticks – The most important is your destiny. It is destiny that brings you to him. And it is your destiny that makes you recognize the real one.
Second are certain signs of the Guru – does he bind you with fear or he maintains you unbound and free? If you are always kept unbound, you are at the right place. In this path, serving the guru is your choice, yet it is also your elevation.
Third is, he needs nothing from you, or he gives you back much more than he takes – if at all. He changes your basic constitution often based on conditionings and fears, to that of stillness and fearlessness.
Fourth is, when you are with him, you will know and recognize a definite change in your vibratory level, a shift in consciousness. The more you are with him; the shifted consciousness becomes yours too. These are subtle aspects one must feel and experience within.
Fifth is non conformity to any habits and mannerisms. He may behave in strange ways often unacceptable to social norms. This is because a Guru who is established in supreme consciousness operates only in the level of cosmic necessity or purpose and not of individual karma. He is only guided by purpose. He has nothing to do with social do’s and don’ts. His operating level is spontaneity, uncontaminated by thoughts. For example; Vyasa impregnated Ambika and Ambalika because the dynasty had to continue. He also impregnated the maid to bring forth an unconditional, wise man called Vidura into the court of selfish royals. Sai Baba often scolded and beat people to drive away evil lying embedded in the constitution of devotees. People, who got offended, never understood the real meaning behind the action. They left him and went away. The loss was essentially theirs. Swami Samarth used to urinate in front of people which distracted many. There are many such examples.
If the Guru is powerful, he would usually be very ordinary or will often act in strange ways. They scold people for their well-being. They need nothing from anyone. True masters would not care about their mannerisms and will have no efforts to make himself socially acceptable. Those who are totally conditioned and bound by social norms and fears will run away from them. Those who stay get all that they can carry.