Sometimes it depends on the text and tune.

"Jesse James," for example, is probably trad, but there may be more than one song with that title. Years ago I learned a version that had the unusual chorus, "Oh, I don't know where my poor old Jesse's gone," etc., etc. Turns out it was revised and copyrighted.

"Elkhorn Ridge" has a trad sound, but I believe it's been considerably adapted.

"Blackeyed Susie" is certainly trad - unless somebody's created a red-hot bluegrass arrangement of it. In that case....

And so on,