One should accept a thing as genuine by studying the words of saintly people, the spiritual master and śāstra. The actual center is śāstra, the revealed scripture. If a spiritual master does not speak according to revealed scripture, he is not to be accepted. Similarly, if a saintly person does not speak according to the śāstra, he is not a saintly person. Śāstra is the center for all. Unfortunately, at the present moment, people do not refer to the śāstras; therefore they accept rascals as incarnations, and consequently they have made incarnations into a very cheap thing. Intelligent people who follow the instructions of the ācārya, the bona fide spiritual master, will not accept a pretender as an incarnation of God.
Ref >> Srila Prabhupada Vani.