Being very old I still use the system I was taught as a teenaged dance-band sax player - I give the key to the other band members by holding up (or down) fingers to show the number of sharps or flats in the (concert) key signature, e.g. one finger up = G (no, not what you're thinking.....) two up is D, one down is F, two down is Bb, etc.. This has the great advantage of not having to shout across a noisy bandstand, which can be open to mis-hearing (like Eb for Bb or Db), and can look a bit unprofessional from the audience's point of view. The other thing which used to help in busking dance- and jazz-bands was that they generally played things in the written key, which in time we all learned - the key the song was published in - although trad bands often simplified keys to suit their limited technique, e.g. C became Bb, G became F, and Ab became the third theme of Tiger Rag and nothing else, ever!