All they’d need to do, is examine the floor of the Red sea, for a trail of human objects and marks on it. Then, they’d need to check if such features exist elsewhere on the planet, under the seas.
Hmm, this might interest you (Seach for Real Mt. Siani): http://video.google.com/videoplay?docid=-4305370740783955461# Esp look at around 15:00...
Ooops, I think I linked a different (but related) one than I was thinking of. You might also find this interesting: http://www.messianic-literary.com/chariots.htm (Chariot found on underwater land bridge through Red Sea — pics included.)
According to Exodus (and rabbinical sources), the whole place of passage was swamped by water rushing in, so I don’t think that would work.