when i had to run an army bar there was a plastic measuring device for telling how many shots were left in a bottle,you just stood the bottle up the right way and put the device to it and read off the scale ,i am sure we weighed the kegs with a special scale ,this also told me exactly what was left in the keg. i tend to agree with others though and would look at my staff ,check that they are not giving credit and not collecting the money. i recently was involved in just this type of thing, a customer had credit on a saturday night ,the barman left the list of drinks on the side of the bar and was off for a week ,another barman found the list and did not know what it was for ,he gave it to the manager who has since found out that half the staff were giving credit without permission and then forgetting to collect the money, the punters in the mean time thought they were on to a good thing.