It depends if it was reported lost/stolen or not. Sometimes carriers forget to blacklist the old ESN/IMEI and just transfer the original owner to a new device/SIM. Call your desired carrier's activation line. You will have to use your social engineering skills to get the number. Call and say you'd like to do an ESN check. They will then tell you if it was reported lost/stolen or if it's clean for activation. If it's clean then take it to the store to be activated, or you could just activate it yourself at that same number if you know what you're doing. Alternatively, you might be able to get a cell phone store employee to check the ESN for you over the phone, but I'd spoof my number just in case. Now if this is a GSM phone it's similar, except you're checking the IMEI to see if it's barred. You could also check it with an anonymous prepaid SIM card. I wouldn't use your own personal post paid account SIM because they could pull up who is using a stolen/lost phone.
If either type of phone were reported lost or stolen, you'd have to change the ESN or IMEI. Good luck with that. Some model phones have been successfully changed, but it is quite difficult to do and I haven't heard of any recent models being successfully changed. It's also illegal so.....
If you're getting a phone activated, most carriers will give you a free phone. That phone you found would have to be very 1337 to even want to bother since the carrier is going to make you sign a contract whether you make use of their subsidy on a new phone or not.
