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.