How can I find PVC pipes in a wall?
Inspection cameras often incorporate a transmitter which allows you to pinpoint the camera's position with a separate sensor. The typical use case would be locating a defect under a slab or in the ground so you know where to excavate, but there's nothing to prevent you from running the camera down the waste pipe and locating it in the wall that way.

Of course you can often save the rental cost by measuring from known points, educated guesses based on the structure, or listening for a snake banging around in the pipe.