How would we know? Would anyone really believe him?
I was thinking about something that happened to me when I was about 10 that made me wonder about this. I was in (Catholic) church with my mom, and a guy walked in dressed like this:
He had the facial hair and a walking stick, and he might have even been wearing a crown of thorns, but I'm not sure about that part.
Anyways, everybody in the church thought that he was crazy. Obviously it was a joke, because if he really had been Jesus, he wouldn't have been dressed in the stereotypical Jesus costume. But how would we recognize the real Jesus? Any miracles that he performed could be easily reproduced by today's magicians. If anything, I think he'd probably be locked up in a mental institution.