There's the genesis block reward (not spendable due to a quirk in the protocol), the address Satoshi used to send the first transaction to Hal Finney (was this really Satoshi? We don't know for sure) and there are a bunch of blocks that we strongly suspect were mined by Satoshi due to their characteristics (mainly the incrementing nonce value).

Something signed with the genesis block reward address's private key would be best proof... But still not perfect, since we don't really even know if Satoshi was the same person over time, or if the same person operated all the Satoshi email/forum accounts.

I don't believe this new pretender really is Satoshi  though (or, hypothetically, any of the people who spoke as "Satoshi") because the writing style is way off. Satoshi was never a mystic. Satoshi never spoke in grand narratives either, just engineering pragmatism.