What indicates that a glass is not beer clean when checking with beer?

A glass that is not beer clean will often show bubbles clinging to the sides after pouring. This phenomenon occurs when there are residues, oils, or soaps present on the glass surface, which create a barrier to the proper release of carbonation from the beer. When the glass is clean, the surface tension of the beer allows it to coat the sides without leaving bubbles; thus, the absence of these clingy bubbles is a sign of a properly cleaned glass.

Good head retention indicates that the glass allows for a proper carbonation release and that the beer is being served correctly, while even head formation is also a sign of a clean glass and good pour technique. Clear visibility of the beer relates to the quality of the beer itself but does not provide information about the cleanliness of the glass. Therefore, the presence of bubbles clinging to the sides of the glass serves as a direct indicator that it is not suitable for serving beer.