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.
At first can we finally say whether satoshi is a person or group?
Well, let's say satoshi is a person and imagine this person came with video where it's all shown how he works on coding, how he posts on bitcointalk and etc + then comes with private keys, all kind of access and etc. I think with combination of all of this, he can prove that he is satoshi.
But why will satoshi try to prove himself? And especially why he will put all efforts in it? He doesn't need it and this task is something that will never happen, so why to discuss on things that will never happen?