Hi, historians. I hope this question isn’t too stupid or insulting. I’ve been reading about Child ballads and I’m curious... how did Child discern or verify that these poems and folktales were actually oral tradition and not made up entirely by the people publishing them as broadsheets? What do modern historians have to say about the origins of the ballads?
The answer to this is complex. First, traditions aren't necessarily very "traditional" - that is, don't look for them all to have millennia-long life spans. Some were/are more recent. Second, the printed/written word and oral tradition have been bound together from the very start. The one influences the other, and back again. Third, traditional narratives - whether transmitted with or without music - change over time and place, so there is no definitive version of anything.
In view of these points, we can recognize that Child was recording popular ballads with the implication that some - perhaps many - generated from oral tradition before being codified by collectors, some of whom further popularized them with the medium of the printed broadsheet. Child was first and foremost a collector. I doubt he even thought it was his obligation to determine the history of each of his ballads trailing into an undocumented oral past (although he noted each's history whenever possible). Some of his ballads may have been written for the printed document, and some of these may have diffused into the popular, oral repertoire. That does not make them any less folklore if they lingered in the oral world that is described by the term "folklore." Other ballads are clearly far older and have deeper roots in the realm of folklore.
I have written on one of these ballads, which is a manifestation of what folklorists describe as the "Lenore Legend," also known as ATU 365, "The Dead Bridegroom Carries off His Bride." Gottfried August Bürger (1747–1794) made this story famous with his 1773 poem, "Lenore," but the story also appears in broadsheet ballads from Britain. It's fairly clear that both the poem and the ballad were independently inspired by similar oral stories, and there is evidence from Cornwall, that at least some of the "history" of this narrative included an oral ballad before it was committed to a broadsheet.
We would need to take each ballad apart in this way, to determine how each figures into the wider answer to your question. We cannot answer your question with a single answer. Rather, the answers (plural!) to your question are different for each ballad.
Source: The Folklore of Cornwall: The Oral Tradition of a Celtic Nation.