The problem is, that its so very difficult to know about the inner purity of another.
Anyone can be happy and gay as long as life goes along his own way
but the man worth while is the man with a smile, when everything goes dead wrong.
An intelligent person can observe a teacher for a long time, while living closely together with him or her, and after seeing no trace of anger or lust, even in ways of speech, they may come to regard that teacher as an Arahant, or on the path to Arahantship.
For example, I heard that at one time, when the Venerable Mahāsī Sayādaw was taking his meals, a heavy ceiling fan crashed to the ground beside him. Had it hit him it could easily have killed him. Apparently, he was completely unshaken by this event, whereas most of us would have been deeply shocked. Such anecdotal evidence about great teachers is not proof of anything very much, but still it is possible to gain confidence in a teacher after a long association.
On the other hand, unintelligent and gullible persons can easily be misled by false teachers. After seeing a few magic tricks they are deeply impressed, and regard their teacher very highly. It is a very dangerous situation for such devotees. If anyone points out that their guru is just a charlatan, they will get very angry and defensive. They donate lavishly, and lose their wealth, perpetuating the myth to all of their friends. Their friends are swept away in the flood.
So, one should not concern oneself too much with the teacher's reputation, but listen carefully to the teaching, comparing it with the Dhamma and Vinaya that one has heard elsewhere, to see if it fits in with it. Best of all, is to practice hard and gain personal realisation through insight, then one will be quite independent of any teacher.
• • • • (Upasampadā: 24th June, 1979)