If you mean the modern US (as opposed to UK and all the other systems) measuring: they don't. Everything is "Cask Strength," and those who make it have their ways (often akin to those of real-world moonshiners, like "the bubble method") for roughly gauging how strong a "swig" is.
— Ed Greenwood (@TheEdVerse) May 31, 2019
It would be fascinating to see the different control methods the cultures would use to determine industrial quality alcohol, as well as drinking spirits. As important a commodity as alcohol is, authentication should play a role in it’s trade. In Calimshan and the Tashalar, they do a "flame test" (thimble of the alcoholic sustance is touched alight under bright lighting, and color and ferocity of flame examined). Most buyers for royal courts and temples do a taste test (that doubles as a poisoning test!).#Realmslore
— Ed Greenwood (@TheEdVerse) June 1, 2019