I posted a similar question a long time ago, and think it was thrown out. However, still looking for an answer.
When busking at a gig, it's often impossible to communicate the key of a piece, due to positioning of players, noise, etc. So, a universal signing system (which I've personally used in various bands for 50+ yrs) is for, say, the vocalist, to show a number of fingers. For example, next number's in E, 4 fingers upwards, next one's in Bb, 2 down. It signifies the number of sharps or flats in the key sig. Simple, and better than shouting across the stage 'it's in G', only for someone to hear'D'.
Now, if we don't know what the song's actually going to be, it doesn't matter, as we can do a 4 bar intro to get into that particular key. This used to happen a lot of the time at gigs, particularly with big bands, during a busking set, with 3 singers, and other musos taking songs segued. So, we'd do, say, a 4 bar turn around, and the last bar would be the dominant of the signed key, bringing the vocals in on time and in key.
Problem is, we never found a foolproof way to signify minor keys. Think about it, if a song's in A minor, we need to know it's not A major, or C (relative major of Am). The last bar of the intro would either be G, if the 'C' key was signed (a zero with thumb and finger), then that wouldn't work well for the singer to pitch, and if the sign was 3 fingers up, then we'd be playing an intro into A major, and the vocals would sound strange after that!
What way round this have any working musos found? It's a very common occurrence in jazz, too - you all know the song, but not necessarily the key someone wants to play it in at the gig.
For those who've not encountered the system, it stems from the Nashville Numbering System, as used by the Jordanaires (Elvis) in thelate '50s. It's an extension of that idea, and is in use by bandleaders.