Which of the following is a sign that a glass is NOT "beer clean"?

A glass that is not "beer clean" can lead to a negative drinking experience, affecting the beer's aroma, flavor, and appearance. Smudge marks on a glass indicate that it hasn't been properly cleaned. These marks can come from oils, grease, or residues that inhibit the formation of a proper head and can also interfere with the beer's carbonation. When a glass is truly beer clean, it should be free of any residues that can affect the beer's integrity.

Excessive foam typically suggests that the glass may not be the right type or is not being poured correctly, but it does not directly indicate the glass's cleanliness. A clear appearance is a sign of a properly cleaned glass, as it should not have any cloudiness or residual residues that would obscure the beer. A cold temperature is essential for serving many types of beer but does not imply anything about the cleanliness of the glass itself. Thus, smudge marks are a definitive sign that a glass is not beer clean and can detract from the overall beer experience.