What I looked for:
1) Is the style a decent one that doesn't have gaping holes in it? Do they teach you mechanically sound techniques? Or do they teach you how to stand in a wide stance, so others can kick you in the junk to demonstrate "toughness?" :rofl:
2) How do the instructors teach the art? Are they patient, understanding, and competent? Are they lacking in instructional abilities? Or are they berating individuals who look for any opportunity to insult the students?
3) Who is the chief instructor? Is his CV a reasonably good one? Is it lacking or dishonest? Or does it have five or more 10th dan certifications on it?
Even if he has only trained in a single system, if the system is a good one, and if he received good instruction throughout the years, then I would have faith in him.
4) Advancement: Are the students ranked in an honest manner? It need not be a direct comparison from one to the next, since advancement should be based on the improvement of the individual. Still, did they actually earn those ranks, or were they just simply given to them after a set period of time? When I see black belts who can't throw a fundamentally good punch to save their lives, then that's where I wonder...
4) Now we come down to price... I'm perfectly willing to pay a decent amount of $$$, as long as I'm getting what I want. I'm not afraid of a $100 / month pricetag, provided that I'm getting quality instruction. But a place asking for $300 / month (and yes, there are such places) is going to be way too rich for my blood, when I know that there are other quality institutions for a more reasonable price.