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.
When all these had happen we can now say everyone is satoshi.
Its really possible because after all there is no way anyone can prove even he himself right now because no one will believe him. A lot of people already claimed he is satoshi but the profile they portrait doesn't really reflect. They weren't as techie as satoshi but the fact that they claimed to be satoshi mean they are not satoshi. Satoshi doesn't have time to convince anyone.