how do you tell if the oil pump is leaking on a 1996 mercury grand marquis

Make sure the oil is full, find a big piece of clean cardboard and put it under the car. Check it frequently, especially after diving for awhile then park it and check shortly after. The cardboard will give you your answer!

The oil pump is inside the engine. If the oil pump is leaking the oil stays inside the engine. If the oil pump is leaking, or failing, you will have low oil pressure.

its internal. what is the issue? low pressure, or a leak on the ground? if its a drip, look at something other than the oil pump

look around oil pump. check oil level and look on ground

l am not sure if you have a light or a gauge but ,with a bad oil pump you will see low oil pressure or none.As for the light it will stay on because it is telling you you have a oil problem.I'd make sure the pan and oil filter was not leaking first.If it turns out it is the pump,pull the oil pan,and the pump will be right in front of you.Be sure to prime the pump when replacing to save dry startup. Best of luck.

it is probably the oil sump and not pump, wipe it off and run for 15 minutes and check again.

It usually is the valve cover.