Iain Grant would use a trick which I think came from the trad jazz world - he'd hold fingers up or point them down with the number of sharps or flats in the key signature, e.g. three up was A and two down was B flat. A refinement was to wiggle them from side to side to indicate he was in the relative minor.

It's more usual nowadays for players to just shout out "Arse", "Bollocks", "Cock", "Dogshit", ... Mozart would probably have done that.