Talking with residents has been the number one piece of advice I've received during interview season. They are in the position you are applying for and can give the best sense of what life is like for a resident at that particular institution. Also, they are more likely to give 'real' responses rather than sidestep embarrassing aspects of a program. But, what do you ask them? These three questions from Ken Iserson are a good start.
Personally, I do not have set questions to ask the residents. I just try to talk to them to get a sense for who they are and what kind of residents compose a program. Are these people like me in a broad sense? Can I see myself working with them (especially if you talk to interns because they will be your senior residents)? Do they appear happy (without directly asking them)? This requires careful listening but can be much more insightful than asking, 'Why did you choose this program.'