Here is an article in an Indonesian web site
http://iphoneall.org/index.php?s=fake
It lists some of the Fake Unlocked iPhone symptoms that they have seen. It's in Indonesian so I'll summarise for you. You can see from their pictures.
Note: This is a different Illegal Unlock to the one with a small circuit board in the tray. In my fake one there is no circuit board as per the ones in the photos. see Malcolm Rayfield's entry in this thread.
1. The phone won't work after 2.2 update which updates the baseband Modem Firmware to 2.28. If you go back to 2.1 the Baseband stays the same. So if you have a fake one, stay on 2.1.
2. The "carrier" option is missing in the settings page. This may also be missing with some sim cards as "markogin" says in this thread, so don't panic if that is your only symptom.
3. The phone may not work properly unless "Settings>Nework>Data Roaming" is on.
4. The box it comes in is shorter than normal. It is less than the width of the iPhone. The real ones are 1.5 x the width aproximately.
5. Inside there are modifications that can be seen by a technician but not dummies like me.
6. When you sync with iTunes it will give a message iPhone is Activated if it is real. I'm not sure if this is only for the first time or each time you sync. (because I've never seen it) 😟
7. the other one is the Model number origin. Mine MB048LL is from USA AT&T as "eclau" points out which has no unlocked iPhones. It was bought from HK.
The other symptom is that the guy/girl who purchased it is very ****** off as they have paid extra for a fake. Usually the fakes are cheaper.
Anyway, the shop I bought it from is going to replace it for me with a real Unlock with the next shipment. He claims he was cheated as well. Let's see.