I think that you're best off with reviews of students and a professional resume.

Will the school provide references of former students? Can you call/email them for their comments?

Does the Instructor provide a resume/CV of training he/she's undergone? Do they have formal training as an instructor or are they self taught? Where did they get the formal training from? Military? NOLS?

Also, how long have they been in business?

If they can't, or won't, provide this information, then I'd consider going elsewhere. Also, just because someone's had lots of training doesn't mean they'll inherently be a good instructor.