In an informal situation (fortunately, that's the vast majority of them), somebody who's not sure what key he or she sings a given song in can hum either a snatch or maybe a full verse of a song before actually starting--by the end of the verse, instrumentalists can virtually always figure out the key. In a large portion of country songs, for instance, the key is the note sung on the last word sung in a chorus.

Then after instrumentalists do figure out the key, they may ask the singer to just go up or down a half step (particularly going up to C (no sharps or flats) from B (5 sharps--not the choice of fiddlers, for instance)--though Bill Monroe, I understand insisted on singing a awful lot in B (just to be cantankerous?--or to test his sidemen?--who knows?)

I've always thought that adjusting a half-step should be no problem for a singer. A full step--that's a different story.

But if you want instrumental accompaniment, it's eminently reasonable that the singer should be guided onto a specific key (not in between keys)--if he or she has no idea what key they sing a song in.