Are there visual differences between regular and decaf coffee? I once asked a waiter at a restaurant how he managed not to mix up cups of regular and decaf coffee when bringing them to various tables. He replied that he could tell by looking at the bubbles – in one type, the bubbles linger, whereas in the other they disappear quickly (of course, I can't remember now which one he said was which).

However: I can't find any evidence or research that supports this, but I don't have any reason to believe that the guy wasn't being serious.

Is there any truth to this?
Coffee foam/bubbles is made up of a mixture of proteins, sugars, oil droplets, caffeine etc, and the lifetime of a bubble is highly dependent on the composition of its membrane. Given that decaf is coffee that's been solvent extracted, it wouldn't be at all surprising to see its different composition affecting bubble lifetime. However, no one seems to have conducted definitive studies of the matter. – answer upped from comments as requested.