iPhone 4
You can easily identify iPhone 4 by its glass and steel design. iPhone 4 is flat on the front and back and has a stainless steel band around the edges. The volume up and down buttons are marked with a "+" and "-" sign. iPhone 4 (GSM model) has a SIM tray on the right side of the case for holding a micro-SIM card. iPhone 4 (CDMA model) does not have a SIM tray.
iPhone 3GS
You can distinguish iPhone 3GS from iPhone by its plastic case. Additionally, the serial number is no longer on the back case imprint. You'll find the serial number for the iPhone 3GS on the SIM tray. You can distinguish iPhone 3GS from iPhone 3G by the imprint on the back of the phone. The entire imprint on the back of iPhone 3GS is bright silver (not just the Apple logo).
iPhone 3G
iPhone 3G
You can distinguish iPhone 3G from iPhone by its plastic case. Additionally, the serial number is no longer on the back case imprint. You'll find the serial number for the iPhone 3G on the SIM tray. The imprint on the back of the phone has a faded look. Only the Apple logo is bright silver.
iPhone
You can easily identify iPhone by the metal case and the serial number etched on the back.
