How can I be sure my iPhone is unlocked to any network?

I'm selling a iPhone I own. I erased it first and then I asked for it to be unlocked and at&t completed the request. Then the instructions say 1) back up your phone 2) erase your phone 3) restore from back up. But I already erased the phone and then asked for it to be unlocked. What, if anything, do I need to do to complete the unlock process? Or is it good to go now?

You'd have to use a SIM from a different carrier to verify.

Well the only way to verify as SRASC said, is to put in a different carrier's SIM. If it is unlocked, the phone will accept the SIM card with no issues at all.

Most carriers you can just unlock it on their website if your contract is paid off.

Be sure to remove icloud lock off your phone also.