Walter Pardon et al knew which of their personal repertoires they regarded as folk songs and which were something else. But how did they decide? What criteria did they apply, consciously or instinctively?

Verse form, including metre? That's dropped out of consideration in Anglo-American folksong studies but was very important for Bartok and pretty much everybody else in southern and eastern Europe - and it's a glaringly obvious difference between old ballads and Tin Pan Alley or Dylanoid material. (Though if you go back to the Middle Ages, you find long lyrics with complex verse forms built from short lines much like the norm in rock).