You could actually ask to see the register reports for all the registers. If the place is of ANY size, they have electronic registers and all sales on the night go through them. The manager gets a report from each one, which can show by time, I believe, how much sales were. That's how they can tell if the cash in the till is right at the end of the night, or if the server or bartender was robbing them.
I wouldn't be surprised if they are screwing you. I think they don't expect people to be savvy enough to ask to see those register reports. If you do get to look at them, make sure you see the reports for all the registers they have, and make sure the report shows credit sales, too, not just what people paid for with cash.
Of course, asking to see those reports could cause you to not get booked there again....