Howard,I do not think the word folk song is a judgement,the judgement is in the minds of collectors,who reject something if the singer owns up to having composed it ,but gladly accept it if the singer says he learned it from his grandad.

That's not true of any collector I can think of, who do you have in mind?

The usual procedure is simply to make sure you record what you can find out about how the piece came into the performer's repertoire. Go look at a few serious modern collections sometime. Or read a few issues of Tocher or whatever.