Hi Johnny,
I think there are two questions; how to tell whether someone is practicing according to Mahasi Sayadaw's method, and how to tell if someone is practicing according to the Buddha's teaching. It's not so important that someone teaches exactly according to the Mahasi Sayadaw's method, but it is very important that they follow the outline given by the Buddha in some form or another.
Whether someone is a difficult question to answer, since the Buddha's outline is indeed quite flexible, and everyone seems to think their interpretation is not an interpretation and actually identical to what the Buddha taught (i.e. exclusively).
That being said, there are some general principles that, if you ask me, I would say are fairly standard among Theravada Buddhists as being ways to tell whether someone is actually following the Buddha's teaching.
If, for example, someone lets it be publicly known that they are an Arahant, or some other form of enlightened being, it is generally accepted that they are not. There is actually room given for this in the Visuddhimagga, which I personally agree with:
But how then, does someone with cankers destroyed declare himself thus: “I am one whose cankers are destroyed?” Why not? He declares himself when he knows that his instructions will be carried out.
Vism III.63
In other words, one might tell a student whom one thought it would benefit; even such a case I would have to say unusual - the Mahasi Sayadaw himself gave a unequivocal answer to this question:
A true follower of the Buddha should have few desires. He should be content with what he has and he should try to lessen his defilements. He should have little desire for material possessions or attendants. He should not want to speak of his accomplishments in the study of scriptures or in the practice of meditation. He should keep the depth of his learning or his spiritual attainments to himself. A true noble one does not reveal his spiritual insight although he wants to share it with other people. It is only the religious impostor who calls himself a noble one or an Arahant.
Sallekha Sutta, Introduction
The matter is further compounded if a self-proclaimed arahant is still observed (and confirms themselves) to have greed, anger and/or delusion inside themselves. Claiming to not be attached to these states is not protection; an arahant, according to the Theravada, is unanimously understood to be without these three mind states.
It is important to remember that one can be a powerful and experienced meditator and still not be enlightened. One can be a charismatic speaker and still not be enlightened. One can act and speak in a kind, compassionate, peaceful, focused, clear manner and still not be enlightened. The only sure way, the Buddha himself said, to know whether one's teacher is truly enlightened, is to spend a great deal of time with them:
"It's through living together that a person's virtue may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning.
"It's through dealing with a person that his purity may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning.
"It's through adversity that a person's endurance may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning.
"It's through discussion that a person's discernment may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning.
Thana Sutta
And how should it be known? This is also explained by the Buddha, in terms of whether the person is observed to have in them greed, anger or delusion:
How is truth discovered? Here a bhikkhu lives near some village or town. Then a householder or his son goes to him in order to test him in three kinds of ideas, in ideas provocative of greed, of hate, and of delusion, wondering, "Are there in this venerable one any such ideas, whereby his mind being obsessed he might not knowing, say 'I know,' unseeing, say 'I see,' or to get others to do likewise, which would be long for their harm and suffering?" While thus testing him he comes to find that there are no such ideas in him, and he finds that, "The bodily and verbal behavior of that venerable one are not those of one affected by lust or hate or delusion. But the True Idea that this venerable one teaches is profound, hard to see and discover; yet it is the most peaceful and superior of all, out of reach of logical ratiocination, subtle, for the wise to experience; such a True Idea cannot be taught by one affected by lust or hate or delusion."
It is as soon as by testing him, he comes to see that he is purified from ideas provocative of lust, hate, and delusion, that he then plants his faith in him. When he visits him he respects him, when he respects him he gives ear, one who gives ear hears the True Idea, he remembers it, he investigates the meaning of the ideas remembered. When he does that he acquires a preference by pondering the ideas. That produces interest. One interested is actively committed. So committed he makes a judgment. According to his judgment he exerts himself. When he exerts himself he comes to realize with the body the ultimate truth, and he sees it by the penetrating of it with understanding. That is how there is discovery of truth. But there is as yet no final arrival at truth. How is truth finally arrived at? Final arrival at truth is the repetition, the keeping in being, the development, of those same ideas. That is how there is final arrival at truth."
Canki Sutta
That all beings said, it is not imperative that one find such a person who is totally free from defilement; it is important that one find a teacher who teaches according to this teaching and not otherwise, as mentioned above.