I suspect I have a leak in my irrigation pump suction line. I don't suspect the check valve at the entrance of the line. The water supply is a lake. The 2" PVC line comes from the lake, briefly out of the water at the bank, then goes underground and comes up in the pump house. There I have a "spool piece" with unions on each end and connects to the 1¼" galvanized nipple on the bottom of the pump. The pump is a Sta-Rite 3-stage pump.

I would like to know methods for leak detection (the source point of the leak) of a suction line. :)

PVC is easy enough to cut and repair. I would pressure the line first. How far does the pipe travel. Just install new pipe if you can't find the leak. 2" pvc pipe is pretty cheap.