How to Test Pressure Switch on a Furnace. In this video I show 3 ways how to check a pressure switch. The first method is to measure continuity and resistance while the furnace is running. The second method is to check voltage on the pressure switch while the furnace is running, and the third method is to put a manometer in series with the pressure switch to see what vacuum pressure the inducer motor is pulling and if it matches/exceeds the rating of the pressure switch.