Where would I find thr drain for my cental air unit and how will I know if it is clogged

Guessing you are meaning the evaporater drain,
It is either inside-on top or under you furnace/airhandler
You will find a plastic/pcv pipe either 1/2 or 3/4 inch diameter,coming out and running into a drain or into a condensation pump,while ac is running water should be coming /trickling out.
If not you can try suction with a good shop vac,or blowing into pcv with compressed air