How to know that your iphone can be unlocked by gevey sim or not?

I'm thinking of buying a cheap locked iphone 4 or 4S
how i get to know my iphone is unlockable by SIMPLE means

The gevey sim unlock depends on the iPhone baseband or modem firmware, should verify before buying if the baseband is supported by the gevey sim. Some networks may be also very cheap to unlock by imei, factory unlock, so if you find one of those networks may be even cheaper that with a gevey sim