There is an OBD port under the left dashboard. There is an electronic apparatus you can buy that plugs into it and will read error and service codes if something is not working correctly.
The OBDII reader is one of the handiest things you'll have (and not just for the GT). When the "Check Engine" light comes on you can read the code (cause) and clear codes (note 1). Some also do real-time data logging. I got one that plugs in and uses bluetooth to a Android phone for the reader. The app is free (Torque) and the Bluetooth reader was about $20 on Amazozn
(note 1: If you're trying to clear codes, especially evap codes, as part of getting emissions to pass, do not clear codes. Clearing codes erases all of the major systems to recalibrate (e.g., O2, catalyst, evap, etc). That requires the full drive cycle and depending on what driving conditions are needed can take lots of miles in driving normally. The Ford drive cycle is posted on the net and if you target those specific sequences can clear all of the system readiness in under 50 miles). Back to evap: the primary evap code is the gas cap seal, so if it calls for that just replace the cap and run it through that cycle only and it will clear itself without a OBDII clear.