Ask them about what services they provide .... ie what happens when you come to see them.
How often they work - especially if you need a special time or day. (some CDC's are part time/some work only evenings/some don't do weekends etc).
Do they keep a full stock in all the ranges.
Then see how you feel you get on with each one - your relationship with your CDC is important in keeping you on the straight and narrow at times when otherwise you'd waiver.