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.
But at least he has those things that others who claim they are Satoshi Nakamoto don't have, and that's a good start and from there we can validate him if he is the real Satoshi Nakamoto our old members and experts here knows many things about the real Satoshi so they can ask questions and get hints if he is the real one.