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.
To those who do not trust the code. it is really impossible to prove that he is Satoshi because you already deny any possible proof that a person is the real one. Ownership of the address and the Bitcoin in it is proven by the private key so is the identity behind that wallet address owner. So not believing anyone who can move the Bitcoin on Satoshi address being the Satoshi himself is the same as saying Satoshi never existed.