"Well, jump in" says I. I have a couple of rules of thumb that I use to define folk music for myself. The broadest one is that a song has to be at least 20 years old (purely arbitrary) and sung by someone other than the author. This avoids ephemera and singer/songwriters whose only goal is to promote themselves. I can always sing a song I wrote 30 years ago but does anyone care?
After that, I'll get into some of the stuff that Bill Day mentioned earlier - mostly style. As for electric instrumentation, it's really not my bag but but it certainly moved into the tradition in country blues.
There are some songwriters whose music moves almost immediately into the tradition. It's picked up by others and sung widely with few people knowing or caring who the author is. In addition to Craig Johnson, I would particularly point out Bob Coltman and some of Utah Phillips' songs.
Isn't it also interesting that the problem does not exist for the most part with tunes? There are lots of old-timey and Irish tunes being written today that, because of an established style, are virtually indistiguishable from older music.