I saw an article on here a few days ago that suggested you could detect isotopes present in "more recent" forgeries that came about as the result of the nuclear testing age that would not be present on the originals.
You can test the pigments in the paint to see if period-correct materials were used. A lot of forgeries use pigments that wouldn't have been invented or otherwise available at the time.
Assuming there are copies, "the original" is the one everything believes to be the original. If the one actually created by Munch is a different one, it's not worth a whole lot if people don't generally believe that to be the case.
Nitpick: it is a bit weird to talk of the original or even of the Scream. Munch created four versions, two paintings and two pastels (in addition, about 45 prints of a lithograph were made, a few of which were hand-colored by Munch)
It seems we don’t know for sure which is the oldest. One of the paintings and one of the pastels both are from 1893. It’s natural to assume the pastel was a study for the painting, but we don’t know for sure.