The British Association for the Advancement of Science describes a test
that medieval brewers supposedly used to measure how much alcohol was in beer:
To test a fermentation mixture in a brewery, pour onto a wooden seat. Sit in this puddle wearing leather breeches, while drinking more beer. Try to stand up. If breeches stick to seat, the beer will be strong. This method was used by 13th century Ale Conners, mediaeval Customs and Excise Inspectors. The stickier the mixture, the more sugar. This will produce more alcohol, so more duty is payable. Modern methods are more sophisticated, but less fun!
I suppose this would work. But whether or not medieval brewers actually did this, I don't know.