The light only comes on when something is out-of-spec so, you will find a "stored" code if it's been on recently.
And, no, those really inexpensive readers are of limited use. Although you could try one (you must know somebody with a code reader or could try an Auto Parts Store) but it could well say "No Codes Present" even though there are...
The scanner of choice here (for at least the $$) is the AutoEnginuity and it's around $350. If you plan to do your own work on your truck for some time Well Worth the Cost.
Is your truck "chipped" or otherwise programmed? Because that will often put things far enough out-of-spec to routinely set the SES.
If not, and it comes on again & again, you should find out why though. You could head off a problem before it becomes a problem for you!