It is said that if one does not know how to meditate correctly on the nature of mind, then one can fall into the wrong pot.

But if the master has the very special ability of being able to know his student’s mind, he will know whether or not they have truly experienced the nature of mind. Other than that direct mind-to-mind interaction, there is no way to know whether or not they have experienced it, because it is beyond words and concepts.

-- His Holiness Penor Rinpoche