Find out if my iPhone is unlocked without purchasing another SIM
Three methods work for deteriming if an iPhone is unlocked:


You see the one time message from iTunes when it restores a phone that previously was locked and now is unlocked.
You contact your carrier and they will tell you if the phone on your account has been unlocked.
You swap the SIM with one that isn't from your carrier and it works.


Apple maintains a nice article explaining which carriers offer an unlock as well as other iPhone related services. If you didn't buy the phone yourself, it could have been sold originally as an unlocked phone in which case you would need to know this fact or experiment with two SIM to know if that device is in fact one that is authorized as an unlocked device.