All this talk of dowsing and witching; we have several fixed features; a meter box; and 5 waterers, each that we know (or we think we know) are part of one system. So, we have 6 known points. We can witch if we want, but that doesn't change the fact, we have to spot dig the line. It's not that bad to dig around the line at one waterer, see the direction it's heading, go down 50 ft, and dig a slit trench to confirm, and go down another 50 feet. If we do a 5 ft wide slit trench, perpendicular to the running line, vs a 10 ft slit trench, cause we have a better idea with the witchy sticks, great. We still need to get a shovel out, and start digging.
Once we have a layout, you can find valves, and start turning them on, 1 at a time, and decide if repairs are needed/worth it; and then more shovel time.
It would be nice if there was a less manual way, but there really isn't. Just start at one, and get after it.