I think that there are some bitcointalk mods know "real Satoshi Nakamoto". If someone want to prove that he is Satoshi, it can be verified by these mods.
If a second person knew about the real identity of Satoshi then it would have been public by now as you cannot hold a secret like this for a long period of time and the mods nor the admin does not have any idea about the creator.
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.
It is a real possibility, even if someone proves that he is the having the private key with them, it only shows that they are the current holders and not the real creator.