Are you referring to a phone being carrier locked, that is, locked to a specific carrier's network? If so, there is no way to tell from the phone directly (which in my experience is the case with most phones, not just the iPhone - that is, there's nothing on a phone itself that tells you it is carrier locked or not).
I suppose you could try calling Apple support. give them the serial number and ask - but I don't know if that would do any good.
You really need to ask the seller if it is carrier locked, and to which carrier. Then, if you do not want to use that carrier, you will have to contact them about unlocking it (Apple merely processes unlock requests that it receives from the carrier - ONLY the carrier can authorize and initiate an unlock).