Pretty much what you would look for in any other martial arts school. Look at the students (both seniors and rookies) and ask yourself if that's where you want to be some day. Watch the classes and see if you "fit" well with the instructor and his/her teaching style. Some instructors are very focused in their teaching, others prefer a relaxed learning environment. Which would you prefer? Research the instructor, see how long he/she has been teaching and who was his/her instructor, instructor's instructor, and so forth. Most legit instructors can name their lineage back at least three generations. Check them out to see if they really studied with the person(s) indicated or if they are "seminar wonders" or "video masters". Look also for an instructor who teaches both the martial and health aspects of the art to some degree. Ignoring one or the other will only give you a "partial art".