It is impossible to prove that somebody is Satoshi.
I firmly believe that even if real satoshi turned up one day, people will also declare him Faketoshi.
This is so true.
Even if Satoshi presents his privatekeys, signs messages, shows his passwords, e-mail account accesses... it could be someone else who has all these privatekeys, passwords, accesses now.
He would only prove the ownership of these but it is impossible for somebody to prove that he is Satoshi.
It would have be possible if he embedded few things in the first blocks that are unique to him, like DNA, finger prints, body marks, eye color and even hidden signatures with information on who owns them . The good thing about embedded data like that is that they can't be changed (immutablity)