If it is a true factory unlocked phone then the model number will be one of the following:
MD234LL/A 16GB Black
MD237LL/A 16GB White
MD241LL/A 32GB Black
MD244LL/A 32GB White
MD2257LL/A 64GB Black
MD260LL/A 64GB White
If it is a phone that was purchased locked to a carrier and then officially unlocked the only way to tell is to do a factory restore in iTunes and see if it displays the "This iPhone is unlocked" message afterwards.
The above list is correct for US unlocked devices, to see a list of models from around the world that are unlocked visit this link its more exhaustive.