Retired pits are hard to read, best to have a depth map, otherwise it's a lot of legwork. The easiest way to figure them out is during open water when you can run a boat depth finder over them.
Sometimes looking at the banks can help. Generally steep banks or big waste pilings means a sharp drop off, but that's not always the case. They can also be dotted all over the place with waste mounds, which can be a really good place to start, since you can have a lot of depth variation over a very small area.