Ask if the agency is licensed or bonded as an employment agency, if that is required by your state. Keep copies of all important papers, such as your contract and agency literature. Be sure to get all verbal promises in writing. Be wary of an agency that requires you to pay an upfront fee in order to represent you. The company simply packed up and left town.
Most legitimate agencies make money only by taking a commission from their talents' work.
An exception, however, is that you may be charged for your picture to be in an agency book that they send to clients who hire models and actors. Make sure you pay only your portion of the printing costs. Also, ask to see a copy of the agency book before you pay any money. The hours of a model and actor is uneven and sporadic. You will not have the flexibility to choose your own hours.