I'm new to this so I'm taking a stab at it. If it's possable, how would i be able to check an O2 sensor to see if it's the reason for the check engine light on a 1991 Ford F-150 Custom?

 

You could get an indication by tapping into the sensor signal wire with a digital meter set to the millivolt scale. It should fluctuate between approx. 100 to 900 millivolts.
If it doesn't , there could be other causes, but you might at least see if anything is happening.
It would be best to get the code(s) from the computer then attack the problem from there.