Do you multiply To determine the percentage of alcohol a liquor contains?

The easiest way is to look on the bottle and take the manufacturer's Word for it.

However, it's possible to test it if you're so inclined (and have a sufficiently equipped laboratory). The easiest way would be GC/MS, which on a modern instrument will tell you which peak contains the ethanol and how much of it there was. If you know the amount of liquor introduced into the system, then you can just divide the amount of ethanol by the total sample size and multiply by 100 to get the percentage.

If you don't have a GC/MS sitting around idle (they're not expensive by lab standards, but extremely pricey by household standards ... a few thousand dollars for a cheap used one), then you're into quantitative organic chemistry, and may God have mercy on your soul.