I have have a 1997 Astro van that shows no oil pressure on the dash gage. The dipstick says the oil is full. How do I verify that the dash gage and oil pressure switch are working O.K. before I start replacing the oil pump ?

get a mechanical oil pressure gauge and remove the oil sending unit, put it in there and make sure you have pressure.
if it differs from your dash gauge the sending unit may need to be replaced.