Don't rush it. Gather what you can and learn what you can to refute any lame excuses or strawmen he may put up in front of you.
Further, how do you know this guy? Is he a friend? is he "the guy that took you or your friends money" and you want to get back at him?
The best approach is to be professional and non-accusatory. It is a simple matter of typing his instructors name on Google and you'll get huge red BS flags all over the place. So, being a conscious consumer you checked a bit and wanted to know what he thought of it all.
What is his training?
Does he have any affiliated or satellite schools?
Also, take the class and talk with him after class...unless of course he jumps right into his used car salesman to pressure you into signing up.
Slow, rational and methodical is the way to go if you are going to confront the guy.
Good luck. And if that doesn't work remember the heel hook....after all it IS TKD and you know how they like to throw their legs at you.