Yes, the only way to be absolutely sure is to pop another sim card from any other carrier in. Perhaps you have a friend who has AT&T or T-Mo service. You could borrow their sim for a minute to check it out. Or if you just go to any independent wireless store, they usually have a lot of sims sitting around and would probably let you borrow one for a minute to check.

Also, you could call Cingular customer service and just ask if the phone is sold unlocked, but personally I would prefer the sim card check.

Fwimbw, I have a Cingular branded Motorola V400 (quad band), and it was in fact sold by Cingular unlocked.