One day the Buddha suddenly pops this on his disciples:

“Bhikkhus, a bhikkhu who is an inquirer, not knowing how to gauge another’s mind, should make an investigation of the Tathāgata in order to find out whether or not he is fully enlightened.”
(Vimamsaka Sutta, Middle Length Discourses 47, tr.Bhikkhu Bodhi)

How do you know if a teacher is the “real thing” or not? If you’re serious, you’ll try to find out, and not just swallow what you’re told. The problem is, since you’re not enlightened yourself and cannot see into the mind of anyone else, how will you ever know if your guru is everything he is cracked up to be? If you’re a real inquirer, and not just somebody who wants comfortable or pleasant states of mind, you will investigate — and the Buddha tells you how to go about it. The list of criteria that follows is further proof of the Buddha’s common sense and mental clarity, and it also implicitly contains a profound challenge to the seeker who wants to know the truth and not just to be told.

“Are there found in the Tathāgata or not any defiled states cognizable through the eye or through the ear?”

First, when you watch this prospective teacher carefully, does anything strike you as wrong or “off”? — not only the obvious defilements like greed or lust, but subtle attachments, lying, deviousness, narcissism? “Cognizable through the eye or the ear” seems to cover everything done or said, but also gestures and facial expressions. Second, Are there found in the Tathāgata or not any mixed states cognizable through the eye or through the ear? This can be interpreted in at least two ways: Is the teacher’s conduct consistently good, or is there unsteadiness and inconsistency — for example, often loving but sometimes choleric and spiteful? Or is his action sometimes ambivalent in quality — for instance, showering one disciple with so much praise and affection that the disciple is blinded by this excess of welcome attention? I once witnessed how a revered swami, all sweet and chuckly to his audience, viciously scolded and slapped the small boy who brought him his tea because it wasn’t warm enough. This may indicate a “mixed state” at best, a “defiled state” at worst.

Supposing you don’t see mixed or defiled states: “Are there found in the Tathāgata or not cleansed states cognizable through the eye or through the ear? ” There is a bright, loving energy about this teacher, no obsessiveness or insecurity, nothing unhinged, nothing off — but also, “Has this venerable one attained this wholesome state over a long time or did he attain it recently?” One has to observe the teacher carefully over a long time to know that his goodness of character is well and deeply established. The assumption is that no one is simply born spiritually realized; we all have to put in work and effort over time, and it takes time for virtue to become rooted.

When you have found someone impressive and are certain that the admirable character is firmly settled in virtue, examine how fame and prestige have affected him. “Has this venerable one acquired renown and attained fame, so that the dangers connected with renown and fame are found in him?” We have seen in our own time many instances of the corruption of gurus through adulation and the absence of critical scrutiny: sex scandals, money scandals, and drug abuse. If the character of the teacher has blind spots and immaturities, fame and adoration will manifest and magnify them. Does the teacher, for instance, consider himself immune to sin and error, and does he listen well to other people or does he constantly presume that he knows what is in their minds? When contradicted or disagreed with, how does he react? The Buddha is asking us to watch meticulously. I wonder if there is even one other ancient sage with the courage and foresight to point out the liabilities of spiritual celebrity.

“Is this venerable one restrained without fear, not restrained by fear, and does he avoid indulging in sensual pleasures because he is without lust through the destruction of lust?”  That is to say, how is his behavior in private, when he is not constrained by fear of the law or social pressure? The Buddha is in fact asking us to pry: it is not enough to know that the teacher is trustworthy on the surface, because we need to know that he is thoroughly trustworthy — that he is good because he is good, and not because he is afraid of getting caught. We should not take this on faith, and obviously, to know such things we need to observe for a long time. We also need to see if the teacher is partial to people, if he is fair, if he treats all disciples equally: “Whether that venerable one dwells in the Sangha or alone, while some there are well behaved and some are ill behaved and some there teach a group, while some here are seen concerned about material things and some are unsullied by material things, still that venerable one does not despise anyone because of that.” Disciples are going to be flawed and in need of guidance, and if you are going to be one of them you have to know that the teacher will give you the same attention as everyone else and not have favorites. A true teacher does not despise imperfect people.

When you strike lucky and find a teacher you can trust, you develop “faith” in the teacher only as you progress in your learning through his instruction: “The Teacher teaches him the Dhamma with its higher and higher levels, with its more and more sublime levels, with its dark and bright counterparts. As the Teacher teaches the Dhamma to a bhikkhu in this way, through direct knowledge of a certain teaching here in that Dhamma, the bhikkhu comes to a conclusion about the teachings. He places confidence in the Teacher.” It will have taken years to get to this point. “Only now, Bhikkhus, when anyone’s faith has been planted, rooted, and established in the Tathāgata through these reasons, terms, and phrases, his faith is said to be supported by reasons, rooted in vision, first.”  “Reasons, rooted in vision” means “reasons that emerge from the depths of our direct experience”: finding your teacher is not about blind faith, surrender to charismatic authority, or trust in someone else’s revealed truth. It requires careful and thorough scrutiny, in which you don’t just “believe” but verify for yourself that this person is worthy to guide you in the search for the most important things.

I can imagine that a spiritual seeker’s first reaction on hearing this would be shock: It’s going to take me so much time and so much work to find a spiritual teacher! Just thinking about the Buddha’s criteria, it seems unlikely that there could be anyone who could satisfy all of them — and even if such a person were to exist, would I ever meet him, would I be fortunate enough to live in the same century or on the same continent? The Buddha’s own disciples are being provoked: how indeed do they know their teacher is the real thing? In an age of upheaval, when old interpretations are threatened and everyone is confused, it is natural for a person to want certainty, and to crave a “still point in a turning world.” This is no less true of the Buddha’s time as of ours. For us the certainty might come in the form of one of the many churches or scientific dogmas; for his disciples the truth had to be found in one of the sixty-two or more views of life that were being vigorously promulgated. His culture was one where a view usually centered upon a teacher, and students were then drawn to this teacher if they wanted the key to the door of their lives. The Vimamsaka Sutta is utterly characteristic of the Buddha in that it gives his students no easy answer, and throws them back upon their own seeing and hearing. Throughout the Pali Nikayas the emphasis is on knowing for yourselves: there is no shortcut to experiencing for ourselves, thinking for ourselves, and seeing from where we actually are. This involves painstaking, frustrating, risky attentiveness. A teacher may tell us something, but until we see it for ourselves we do not really know it — and when we look into our own lives, trying to see, we cannot pre-know what we will see. The Buddha is deeply aware of the terrible temptations of authority. Because we are afraid of our own solitude, and are insecure about what we see for ourselves, we crave the reassurance and support of a teacher. This makes it all too easy for us to replace our own insight with the words of the teacher, to triangulate our own experience against the fixed points of those words, and never to stray too far from them. The disciple’s anxiety about how her own experience meshes with the teachings becomes a screen that eventually separates her from her own insight, which is no longer anything more than the shadow cast by the teacher. The Buddha’s project requires us to meet our own experience head on and to stare it in the face; for this reason, in sutta after sutta, he has to undermine his disciples’ dogged efforts to squeeze a “view” out of him that they can cling to for safety. He wants us to become spiritually adult, capable of self-reflection and self-correction. It is not that the Buddha’s guidance should count for nothing with us, but that that guidance can speak to us only if we are truly engaged, from moment to moment, with our own awareness. Fearful impatience drives us toward quick answers and clear teachings, but we need to look at that impatience too and ask what it really is.

Vimamsaka Sutta ( Majjhima Nikaya 47, tr. Bhikkhu Bodhi)

https://suttacentral.net/en/mn47