1995 cavalier z24 2.3L engine. Check oil light was on for about a week and I noticed that the level was about a quart down and I was do for a change. Did it, new filter and oil. Filled to correct level and light still on. How do I tell what it is, oil pressure, bad oil pump, bad sensor?

You probably need to have the "Check Oil" light reset.

The oil level sensor is either inoperative or out of range. Replace the oil level sensor, located in the oil pan, and the light should go out. (You'll lose your fresh oil change so you may want to wait a while and check the level frequently.)
Rarely is the sensor harness at fault with this message.