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Also with the breaks, CDMA model of the iPhone 4 is exactly the same as the 4S. :p

Except for that little SIM tray on the 4S as the give-away. :D

For all the other convolutions, this is really the simplest/easiest mechanism:
Look at the right edge, the 4S has two black lines and SIM tray.

Two lines and no tray = CDMA iphone4, One line and SIM tray = GSM iphone 4.
 
Except for that little SIM tray on the 4S as the give-away. :D

For all the other convolutions, this is really the simplest/easiest mechanism:
Look at the right edge, the 4S has two black lines and SIM tray.

Two lines and no tray = CDMA iphone4, One
line and SIM tray = GSM iphone 4.

If the phone is Verizon it has no SIM card tray
 
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