You've received some great advice/info in the other responses. My tip: Drop in without an appointment.
When I was looking for childcare for my then unborn son, the best advice I ever got was to drop in unexpectedly on the centers you're looking at. You'll get a *really* good feel for the place and the people. It's easy to make everything be just as it should appear when parents are scheduled to come in for a visit. The *true* test is how the staff responds/reacts when you surprise them by just dropping in. Are they comfortable with you showing up at any time? are things disheveled? are people scrambling to make things look right? or are they perfectly happy to show you around and answer your questions on your time?
As a side note, I've given that advice to every M. who's asked me about choosing a daycare/preschool. Some moms were VERY surprised how much they learned about the center by dropping in unannounced.