Just ask for the imei ahead of time & check w/ the carrier to make sure there's no money owed on the phone (financed phone) & that its not blacklisted. W/ T-mobile you can check here:
http://www.t-mobile.com/verifyIMEI.aspx not sure about the other carriers. If it was an unlocked phone bought straight from Apple, I'm guessin you could call them to check the imei.