One old criterion of 'folk' (from expert collectors) used to be that if the author of song or composer of tune were known, then it wasn't folk. That's not exactly the case now because research has turned up the authors of practically one out of every 10,000 real folk songs. But if you know who wrote it or composed it the chances are very, very small that it's real folk. It has to be traditional to be real folk, and the traditional singer had to learn the song and/or tune from someone else, not from a printed source.