How do bars keep track of inventory?

There are multiple ways bars can keep track. A common piece in any of the methods though is to track sales through the POS. Tracking needs to include how many of a liquor/beer/wine is sold, how many ounces that amounts to, how much money was collected for sales and how much should have been. If you have that data, inventory becomes as simple as measuring how much money you collected against how much was rung in and how much is missing.