Banjiman, I'm really not a purist in any pure sense of the word. A REAL purist wouldn't like trad music played by a rock band ;^)

I can, however, tell the difference quite easily. And I got there by listening and playing. If that method doesn't work for you, I don't know what else to say. I guess I'm not the person to delineate it for you, other than to fall back on the process thing -- where did the song come from?

There is no sharp dividing line. There's stuff that's definitely in one place and stuff that's in another, and there's even more stuff that's somewhere in between. The real purists might well disagree with me about that, but I'm not really part of the whole academic/musicology side of things. Just a musician who knows what I like when I hear it.