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 agree i think proving that you are Satoshi was almost impossible especially today,
Everybody who will claim that they are Satoshi Nakamoto would just end up becoming a faker one, There is already no evidence, in my opinion, to claim that you are Satoshi.