What is one method to check if a glass is beer-clean without having beer in it?

The droplet/sheeting test is a reliable method for assessing whether a glass is beer-clean without the need for any liquid inside. This test involves observing how water droplets behave on the surface of the glass. A properly cleaned glass will cause water droplets to bead up into uniform, rounded shapes that hold together without spreading out. This is indicative of a clean surface that is free from oils, residues, or contaminants, which would otherwise cause the water to spread and form a thin film.

On the other hand, a glass that has not been cleaned properly may show droplets that spread out or leave a film, suggesting that residual oils or soaps are present. Thus, using the droplet/sheeting test provides a visual confirmation of the cleanliness of the glass, ensuring that it is suitable for serving beer, which is particularly sensitive to contamination.