No contracts: Payment should be on a month to month basis, with no long term contract or payment such that you may quit at any time if you desire. If they ask you to sign a contract, run (do not walk) to the nearest exit!
Costs should be reasonable: Be sure that the dues structure is straight forward, easily understood and readily available, and that the costs are in line with other schools (or hopefully less!). All costs (including uniform, any organizational fees, examination fees, any texts or manuals, etc.) should be clearly delineated.
Instructor Student Ratio: ask what the student instructor ratio is, particularly in the children’s classes. The classes should be a manageable size that allows the instructor to monitor the student’s development.
Instructors and students treat each other courteously and respectfully: If the instructor is abusive or overly militaristic (e.g. shouting at students in a denigrating or derisive manner, or demanding tortuous exercises from students) they are attempting to prey on your insecurities. Just politely (but firmly) leave the premises.
Is training open to all observers: You should be able to observe all classes without having to join up first? If not, they are trying to hide something.
You can speak freely to any of the instructors: If not, they are trying to hide something