Here’s what happened with me.
I had no interest in qigong (though I was interested in taijiquan, which I didn’t know at the time basically integrates qigong into itself). A friend of mine was going to this workshop and invited me to come along. We hung out so much anyway, so I just said sure.
In the full-day workshop, the teacher took us through about 8 or so exercises, each practiced for about 30-45 minutes. He lectured a bit and there was time for questions (I did not ask any).
Here’s the thing, pretty much as early as the first exercise, I felt striking and unfamiliar sensations. We did what I now know was a fairly generic ‘qi ball’ exercise, and I was struck by a palpable feeling of magnetism between my hands. (Various people experience the sensation of qi in different ways: magnetism, heat or cold, a wind-like sensation, numbness are common.)
There was one moment that made the biggest impression on me. The teacher gave us the instructions for another exercise, and as I started to do it, my feet began to feel cold. After I noticed that, the teacher said, ‘You may start to feel a cold sensation in your feet.’. For a natural skeptic like me, that was a big deal. Prior to that, I’d thought to myself, ‘Okay, this could all just be some form of suggestion.’. But as we all know, suggestions generally work by coming before the suggested experience. If it comes after, it’s not a suggestion; it’s a description. 
So, the combination of strong physical sensations and independence from potential suggestion made a big impression on me. At the end of the day, the teacher said ‘If you want to lay a foundation of qi development, you should practice these exercises for 100 days.’. I went home, marked out 100 days on a calendar and did just that.
One thing is: during my solo practices, the sensations were nowhere near as intense. A group of people practicing together generates a qi field, I’ve been told. And if one or more of the people in that group has a very strong practice, then that field will be stronger.
So, I think it’s not that it’s vitally important to have a teacher or a group, but it’s easier. It’s easier to have a sense that you’re progressing and that anything is happening. My 100 days of follow-up practice were apparently detectable by my teacher, but subjectively, I didn’t feel that much.
So this gets to your actual question: how to choose a legit teacher. I’d give three criteria:
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You feel some kind of obvious qi sensation when they guide you through an exercise or do a treatment on you.
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They give clear, objective descriptions of the phenomena they’re teaching.
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They seem basically ethical and non-‘culty’ about their teaching. It’s all natural stuff even if people are not so familiar with it. They don’t have to act like they’re more important than anyone else. Just let the practice speak for itself. (That’s my own preference. I don’t like charismatic bullshit.)
There is not ‘ONE TRUE TEACHER’. A teacher (or a student) who spoke like this would meet my criteria for rejection. This is a natural phenomenon. Just like there can be 500 dance teachers, there can also be 1000 great qigong teachers. No one owns the air. Or the energy.
My own teacher, after reaching adulthood, journeyed around China to seek out and learn from reputable Qigong teachers, of which there were many. I’d recommend the same approach. Study with a number of teachers and learn different things from them. Just make sure that whatever the important instructions are, you’re following them. And for any given method, give it a reasonable amount of minimum time to have an effect.
Okay, that should do it.
wow, just wow… I’m impressed that you can handle it.
And your still seeing results?