Sure, moonshine is strong. But back in the "old days," if you really wanted to get ahold of the high-octane stuff, you needed to look for the XXX symbol. Until modern moonshiners began putting their hooch into glass canning jars, moonshine was divvied into clay jugs. These clay jugs were marked with an "X" each time the moonshine was run through the still. The XXX symbol meant the hooch was run three times through the still, making it the strongest possible white whiskey [source: Flask]. Essentially, the XXX served as a warning about the jug's content. Moonshine outfitted with three Xs was probably 150-proof, which is about 75 percent alcohol [source: Hanson].
Today, some distillers claim they can make moonshine that ranges near 100 percent alcohol. But you'll either need to distill the moonshine yourself -- or take the distiller's word for it -- to really know how strong it is.