To be fair, registers aren’t used exclusively by one employee. At grocery stores, when someone goes on break, someone coming back might take their spot. That happens multiple times a day. And for restaurants, almost every employee uses the same few registers.

The only way to know who is stealing is to either watch the drawer or to count the cash every time the employee walks away. Of course, most places go with the former as the latter is too time intensive (especially during a rush). The better solution (IMO) is to just count at the end of the day, and if there’s a discrepancy, mark it down. If it happens multiple times, you can compare who was at that register for each of those days and narrow it down.