when playing for (contra) dances, the next tune in the medley is usually signalled during the "b" part of the tune before it...
and the changes usually co-incide with particular dancers reaching
a certain position on the floor for progressive dances...if the band
expects to play 4 tunes, they (we) will change tunes when the head couple gets 1/2 way down the line, and again when they reach the bottom of the set. If the band has pre-agreed upon set of tunes,
the next tune will be designated by the title, or a nick name for it
for instance for the tune "smash the windows" the band member might just call out "smash"