If the IFD is at software release 10.2.6.1 and prior, the serial number can be located in maintenance mode under the Hardware page. The link below will provide instructions on accessing maintenance mode if you are unfamiliar. If the IFD is at software version 10.3.0.2 or later, the serial number can be found in the Software Status page in Flight Mode. Regardless of software level, the serial number is also located on the data plate on the side of the IFD.

IFD MAINTENANCE MODE

10.2.6.1 and Prior

10.3.0.2 and Later

Unit serial numbers can also be found in the paperwork that came with the unit. 

The FAA Form 8130-3 should be kept with the aircraft maintenance logs, and the IFD Serial Number can be found here:

Alternatively, the logbook entry for the installation of the IFD should contain the IFD Part Number and Serial number when it was installed.