Speak with artists who've had personal experiences or shows with the dealer and find out how well their art sold, how problems were resolved, how quickly they got paid, how well the dealer responded to their needs, and how straightforward the overall relationship was. Sometimes you hear a variety of stories during the course of your research, both good and bad, in which case you have to weigh the plusses and minuses and decide for yourself. Usually, however, the consensus opinion is pretty clear cut. Investigate any promises that dealers make about how much art they think they can sell, how fast they expect to sell it, and what they can do for your career. Once again, ask artists who've shown with them whether similar promises were made and whether or not they came true.
Younger artists are particularly vulnerable here.
They get so excited about having successful shows that they dispense with double-checking and barge blindly ahead. They feel great for a while telling everyone they're having shows, but if they've made poor choices, they pay for those moments of excitement for months to come. On the flip side, some artists try to protect themselves against every possible negative outcome with a myriad of legal requirements and restrictions that dealers must obey before they can host shows. This is acceptable behavior when you're famous, but when you're not, the best way to go is to keep things as simple as possible. The closer you come to a handshake, a smile, and a basic one page contract, the better.