Gawd, another one of these folking holey wars!

How about, "its folk music if anyone you think is folk likes it"?

Or, "its folk if you can sing it without a synthesizer in the background"?

Or, "its folk if you can imagine June Tabor singing it"?

Or, "its folk if you can transcribe it into midi"?

Or, better yet, "Who cares, just sing it"?