When comparing the GSM and CDMA versions of the iPad 2, the most obvious difference is the microSIM slot.

The microSIM slot is located in the upper left corner of the GSM iPad 2. The case is machined uniquely in this area on GSM models to accommodate the SIM ejector mechanism and the SIM tray.

In contrast, the CDMA version doesn't rely on SIM cards of any sort and uses the circuitry on the WWAN board to relay user information. This adds up to an empty space in the outer case above the WWAN board where the microSIM slot was installed on the GSM version.