Since Some bloke mentioned Steve Roud, I think it a pretty good first approximation that, if a song isn't in the Roud Folk Song Index by now, and is in English, it isn't a traditional song. :-) Then we just have to decide if a song needs to be traditional to be a folk song. (Hint: it does. :-p That was easy. :-p)

It's also worth saying that revival singers do have a role in the preservation of folk songs. If it weren't for revival singers, most traditional songs (other than children's songs) would be truly dead. But I would analogize revival singers to scholars of dead languages -- Latin, Sanskrit, Classical Greek or Hebrew, Old or Middle English. They do tremendously useful things; there is immense value in those old writings, and they are preserving them for us and making them available for us. But while they preserve and make available those old things, they are not actually part of the culture that produced that language and those writings. They are a sort of a bridge back to those old, valuable things. But they cannot dwell on the other side of the bridge.