Every morning just check each pump. Pretty basic stuff and that would mean easy-to-discover skimmers never sit for > 24 hours. You could then assign liability to undetected skimmers after a window of time.
Open known-clean pump. Take picture of inside. Give picture(s) to worker, tell him to look inside every day and compare to picture(s). If the inside of the machine doesn't look like the picture, call manager, and put "Out of order" sign on machine.
Every morning just check each pump. Pretty basic stuff and that would mean easy-to-discover skimmers never sit for > 24 hours. You could then assign liability to undetected skimmers after a window of time.