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.
I think that makes sense. However, not fully. It is impossible to ultimately prove that Satoshi is Satoshi, I agree with that. But there can be a certain distinction about various candidates, and a person making a signed message would at least be someone worth taking seriously, and someone who will generally be considered Satoshi. And I think that then this person can get confirmed by some others that might know the real Satoshi from the early times, this person might be able to clarify some things about Bitcoin in a way that will differ from the thoughts of others... And so there'll be a pretty good chance that this person is Satoshi. I, however, think that it won't happen.