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 is possible all of the proof that he would show is legit so why would we doubt him?
If that person could open up all the old e-mails or Satoshi's account+private keys then why should we doubt him?
But this scenario seem's impossible to happen after all Satoshi vanish wothout a trace so that mean's he doesn't have a plan to reveal his true identity.