The Bhagavad-Gita has a section about discriminating between the divine and the demonic.

ultimately it is really not that hard. Evil can't really hide. Eventually it comes out. False masters will, most often, commit actual overt crimes, not merely even unethical actions. It is the nature of such a powerful deception, that it so captivates the charlatan, that they become real sinners.

By that I mean, very tangible and overt things - tax evasion, sexual impropriety, gambling, drinking, lying, etc. etc. etc. The whole range of evil attends those who commit fraud about God.

If you are around someone who claims to be a Master, yet see any of this kind of egregious behavior manifested, you are in the presence of a charlatan.

Real saints exude peace, kindness, humility and all the divine qualities. They can engage in a few behaviors which are confusing to our limited understanding, but 99% of their actions will always be very overtly kindly, loving, saintly, etc. etc. etc. The behaviors which may be confusing are because of things unseen. A perfectly nice gentleman could approach a saint, and, out of nowhere a saint might yell at them...but that might be because they are planning a robbery in their thoughts. There are things beyond our perception that saints act upon. But again, those are rare deviations from what will always be very overtly saintly behavior.

As Swami Vivekananda joked, "in East Bengal, the avatars grow like mushrooms." Saints don't grow like mushrooms. True saints are quite rare. In his lifetime, Gandhi only recognized a handful of people as true saints (Swami Vivekananda being one of them).

So, since many people do claim to be saints, it falls to reason that the vast majority of them are actually frauds of one kind or another. That is specifically in the realm of people who claim to be "perfected Masters."

There are indeed a large number of people who are sincere spiritual people who are spiritual teachers. There is just a difference between a very spiritual monk who teaches and a truly God-Realised soul. God-Realised souls are quite rare indeed. And, among those claiming such a title, only a small percentage are...leaving the vast majority of those who claim such a title to be, in fact, frauds of one kind or another.

"Truth Alone Triumphs" - the Vedas