I can't say this is 100% but it has worked for me a few times.
For those who don't know, well witching isn't witchcraft. It is a crude method used to locate water flow underground.
My method and my grandfather's aren't the same. He could take a wishbone shaped branch, (Barely holding) with his hands on the inside ends, wrists facing out, the point of the Y facing forward, he would start walking. The stick would be level to the ground. As he approached water the stick would start moving until the point (end) would work it's way pointing to the ground. I could never do that right.
What I do has worked for me - most of the time. Don't know if the moon, temperature, moisture level, had any play in it or not. I just never kept a log. Some of my friends I made well witchers for swears by them.
So here's what you need. Two very small diameter straight copper pipe pieces 3 7/8" long. Two coat hangers cut with a 90 degree angle, one end just under 3 7/8" long and the other end 14 3/8". Insert the short ends into the copper pipe pieces. The copper pipe pieces serves as a holder. Holding the two pieces in each hand level slowly start walking. When you cross over moving underground water the two ends will cross. I have a well in my back yard. To prove it true I took a witcher and walked a circle around my well. When they crossed I marked the ground. Kept walking, marked again. Turned around and when I got to the first mark they crossed again, moved on to the next mark, they crossed again. I read that the science behind it is water moving underground creates a form of static electricity or a magnetic field causing the wires to cross. Not sure which one. Pretty neat huh?