Has the song survived the test of time?

Does it reflect a specific cultural milieu?

Are the many variants of the song?

Has the song been collected from a traditional source?

Is it accessible so that it can be learned more easily than an art, show or jazz song?

Has it been sung by many people who sometime change it along the way?

Usually, a folk song comes from an isolated community, most often rural.

A song can be written in a folk-style.

If you can copyright it, it probably isn't a real folk song although many folk song collectors have attempted to copyright public domain material. This was done
a lot in the Sixties.

General popularity of a song doesn't make it a folk song. If you don't believe that,
attempt to take a popular song and change it without eventually getting sued.

This is the flaw in thinking about ASCAP, BMI and commercially licensed songs, they are not part of the folk process of change and variation. This is what separates the folk song
from a commercial product whereby the author/composer receives compensation
for the use of the song.

In time, a commercially written song may become a folk song if the author/composer
is forgotten and the song changes with many variants.