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.
why not possible? I think if Satoshi Nakamoto really came back, maybe he would do something different from what Satoshi is doing now, because I'm very sure if we make something, there will definitely be different methods that other people don't know to prove if it's true we what made it, maybe the real Satoshi Nakamoto would also do such a thing if he wanted to prove it.