Well, first of all, get repair manual for your car

The test is simple, there is a fitting on the engine block that allows you to screw on a pressure guage to measure the main oil gallery pressure. The only thing is that you need to find out where is the fitting. A repair manual will help.
Also it could be a bad oil pressure switch/sensor (if equipped) as well. It is very simple to diagnose if the switch is good or not. Again, a repair manual will be very helpful. Good luck!