Testing oil pump and check valves

[Legacy37]

My old 98 gti has an oil leak into the engine problem. Filled the engine up within a week last year. Seems I have the shut off valve on tye oil pump feed line and it has kept the oil out of the engine. I now assume I have a leaking oil pump or check valves. How do I confirm this? how do the check valves work? At nearly 80 a gallon for the good oil, I want to waste as little as possible

thanks!