Yep, the Cobb AP can read the CEL code(s), tell you specifically what the code(s) is/are, and turn the light off for you. No need to be paranoid. As a backup plan, you can also take your car to your local Auto Zone (or similar auto parts store) and tell them that your check engine light is on and they will be able to run a quick diagnostic test (usually for free) and tell you what code(s) is/are causing the CEL. To clear the light yourself, just unplug your car battery and step on the brake pedal for a few seconds until all the power is gone. This will reset your ECU and the CEL will be cleared.
Good luck with your possible purchase!