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 [...]
This is why I think Satoshi is not a group of individuals but a single talented person.
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.
If someone can easily get the private keys of Satoshi, bitcoin will be worthless by now. They could just dump all his bitcoins. And if someone comes forward with a signed message from Satoshi's bitcoin address, I will believe he is the real Satoshi.