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.
Satoshi never showed up but he at the same time make sure that nobody else can steal his work and claim to be Satoshi. Many tried to prove themselves as satoshi but failed miserably since there is nothing intrinsic to satoshi that somebody can show and claim the prize. Lets wait and see if something good comes out regarding real Satoshi.