Use the wallet keys or PGP key to sign a message and we'll believe Satoshi.
The whole bitcoin system is designed around psuedo anonymity, cryptographic messaging and private keys.
The only way to even start to prove your identity is with a signed message.
I agree that this is how it should start, but honestly, I don't think that it would be an ultimate proof for me. It would prove that this person was very close to Satoshi psychologically (since Satoshi entrusted this person with the keys) or literally (so this person could steal access). But not that it's necessarily Satoshi Nakamoto. Craig, for instance, might succeed in opening the wallet soon (although I doubt that), but I would rather believe that the real Satoshi died and Craig stole his ideas and money. So to sum this up, signing a message is necessary, but not sufficient proof that a person is Satoshi, at least for me.