Attorney-client privilege can make this sticky sometimes, but someone who can act as a reference and say how the lawyer deals with clients will help cement your decision.

If the lawyer won't give you the names of clients, ask for professional references, perhaps the names of lawyers to whom he makes referrals. When you call those colleagues, do not identify the person who gave you his name at first, saying, "I was told you could be a reference for my attorney. I was wondering who you consider to be the best attorneys in town." If your lawyer's name comes up, then ask why the reference feels that way. If it's not on the list, ask why not.