The true guru is the person who says that Krsna, God, is our true friend. He does not pose himself as God or tell us that we can become God.
Whether the guru is a genuine avatara must be determined through the sastras. An incarnation never says, “I am an incarnation.” Nor does an avatara canvass for students. Because of his superior qualities, he is automatically accepted.
The foremost qualification of a guru is not that the guru establish his superiority (whether by magic tricks or whatever) but that he establish the superiority of God and be able to arouse the dormant love for God in the hearts of others.
The most important celebration for a devotee to take part in is his own acceptance of initiation from the Spiritual Master. The fire sacrifice marks the climax of the initiation ceremony, and is followed by joyous singing and ecstatic dancing before the deities.