Up here our 13' dug well had been filled in with rock. I noticed it because there were two apple trees growing on this small rise, a a small pile of stone and rock. It just seemed right and I took all of the rock out. We used it for a few years. Found a clay pipe at the bottom. It was a circular stone well that went down a foot into a chiseled piece of bedrock before they gave up. I think I've found another on our neighbors property. Another neighbor has a 25' well he still uses. It was filled in with rock and he was lifting each out 1 by 1. He went to step on one and it suddenly gave under his foot. It went down another 15 feet.
I look for something that does not belong to the natural lay of the land - brush, a mound, etc. When they dug it, a lot of dirt came out of the hole and went somewhere- that's where the mound comes from.
Good luck and be careful. Try a diviner, or do it yourself- nothing to lose.