There is no way to determine model when purchasing from Apple.
If you request an unlocked iPhone, you will get the model they have that is unlocked. Which currently is the GSM only version. So it will not work with CDMA carriers if any still remain in Europe.
Note that the Warranty will only be valid in the U.S. and should anything happen to it, you will need to take it back to the U.S. to get it repaired.
Other than that, just specifically request the unlocked model, and only buy directly from Apple.
iPhone's bought from anywhere else, are not truly unlocked, and most often than not will lock to the first U.S. carrier used to activate them, and cannot be activated on non U.S. carriers.