At my last place I opened up the well and dropped a weight on a string down the hole. I listened for when the weight hit the water and stopped letting the weight down. I raised and lowered the weight a couple times, listening for it hitting the water, to be sure. Then I put a knot in the string at the top of the well. I hauled the weight up and measured the string from knot to weight
@mushrj
The other important measurement is how deep the actual water is. So once I knew how far down the water is, I used the weighted string again and played it out until it hit bottom. I put another knot at the top of the well, hauled the weight out, and measured the string.
This gave me the depth of the well and a way to measure the water in the well (at that place the well did not produce as much water as I would have liked, so water measurements helped me track available water.