ONE/ we had a math genius who could tally 7 items, include tax, and be ringing it up short at the same time as she was telling you the correct total. Then she'd clear the drawer out of any extra cash. She always made sure that there was no register readout pointed at customer.
TWO: we come in and pull a 20.00 when no one is looking. If it does not show up later as missing that is a big clue. Same with adding a 20.00 if it goes missing.
Three: we had people who just took 40.00 and you did not know because we had 3 people in the drawer.