HOW do I test this?

do u have a gauge or idoit light?

They are normally closed and pressure opens them. The light comes on because it grounds through the switch. If you have a guage and a light, you could simply disconnect the light wire from the switch and use the guage only, but most vehicles these days use a sender (potentiometer) to give the gauge (really an ammeter) a reading. If you really want accuracy, install a real gauge with a line from the oil gallery into the passenger compartment.