48:30 the servant brings in a poisoned drink. The servant tries to fool Cleopatra into thinking she has taste-tested it, by "sipping" it. Then the servant wipes the rim of the cup. Someone deduces this must signify that the cup has been poisoned.

I don't get it. a) seems like a fairly natural action, or common sense, whatever. If I'm food-testing for the king, I'm going to be wiping off his glasses. b) why would that imply poison

Maybe it was explained in another scene.

All I can imagine, is that the servant dribbled liquid down the cup, because she was not swallowing. That seems really weak. People dribble all the time, and stuff like lipstick leaves residue.