John Minear,
Another approach would be to look at a collection such as Sharp/Campbell, selecting examples of ballads that have distict Scottish and English variants back in England. This could easily be done using Child. Then look at how distinct American versions have evolved and in this way draw up some conclusions as to how long they have likely been in oral tradition in these distinct forms. It could then be followed up by selecting versions of the same ballads in other American collections like Flanders. It wouldn't necessarily give any concrete proof but would possibly make good circumstantial evidence. Try, for instance The Cruel Mother, a ballad that seemingly originated on a London broadside but evolved into distinct versions in England, Scotland and Ireland.