Just as a restaurant's or bar's bathroom clues you in on how well kept the whole place is, so might the look of beer poured in a glass. Thrillist notes that bubbles down the sides of your pint is a sign your glass is not clean.
Photo by Smabs Sputzer
Instead of rising to the top, the carbonation gets stuck on the sides because of some type of debris -- dirt, grease, or sanitizer (if you're lucky). Congratulations. You will never enjoy draft beer again.
A commenter on the site adds that iit's not really that the bubbles stick to the debris on the glass, but that "the debris provides a nucleation site". Whatever. We just want clean glasses.
For more ways to tell the quality beer bars from the dives, check out the link below.
8 Simple Ways to Tell If a Beer Bar Is Serious [Thrillist]