How does the instructor or instructors conduct themselves? How do they teach material? What's the student to teacher ratio?
Do the students appear to be enjoying themselves, even if there is a high level of discipline?
How is the movement of the instructor or the students? If you've been around martial arts for long enough, you should be able to tell if the movement is equal to the belt rank, unless it's a system you're totally unfamiliar with and even then, there should be clues you can pick up on.
Are there 10 year olds walking around with BBs? Unless it's a school that has a junior black belt program.
Were you told that if you paid x amount of dollars that you could have your black belt in 2-3 years?
The list goes on.