I just read somewhere that the Dalai Lama said that a person should observe a likely teacher for three years before deciding to become their student. Who knows, the teacher could be misrepresenting Buddhism. But this advice might not help since even those who wish to become students are most likely unable to distinguish a charlatan from a sage.
According to some canonical sources, we live in the age of counterfeit Buddhism (pratirupakadharma) in which Buddhism becomes corrupt, including its teachers. Even if someone reads the section in the Sutra of Perfect Enlightenment about finding a genuine teacher chances are it will not be of help. But I think I have found a pretty good way to tell corrupt Buddhist teachers from those who are not. It has to do with the teachings of pure Mind, or if you prefer, clear light Mind or Buddha Mind. Since all of Buddhism rests on liberated Mind which has been utterly purified of adventitious perturbations; which is the absolute substance, it stands to reason that any teacher not teaching pure Mind almost, ad nauseam, should be considered suspect as not being a good teacher.
When you consider many of the texts of early Zen there is hardly a page where pure Mind is not explicitly or implicitly referred to. In fact, there is no way to explain the almost bizarre behavior and words of Zen masters such as Joshu (Chao-chou, 778–897).
A monk asked, “The others say it [“suchness”] with their mouths. Master, how do you teach it to the people?” Joshu banged his heel against the stove. The monk said, “So that’s what it is, isn’t it?” Joshu said, “What a fine understanding of my heel!”
A monk asked, “Master, what is your great mind?” Joshu said, “There is neither great nor small.” The monk asked, “Ah, so this is your great mind, isn’t it?" Joshu said, “If there were even a bit of this [distinction], it could never be it.”
Joshu, as we can tell from the above, is quite capable of explaining exactly what pure Mind is but he could always recognize those who were asleep, caught up in concepts about Buddha Mind who would never personally experience it. Yes, he was a good teacher.
For the prospective student of Zen, today, who is not uncomfortable with a real Zen teacher who pushes them to the limit of their being, forcing them to leap out of their skins, such a teacher, like Joshu, is a lucky find. No tears will be wasted. Most, however, will never come across such a great teacher like Joshu; and most Zen teachers these days could not care less about teaching pure Mind; its great importance in Buddhism. What a student who is seeking a teacher can expect to find is a mediocre teacher who teaches zazen, or as one Zen master described it, ghost sitting.