Satoshi is known to have mined the first several blocks for Bitcoin. Each of those blocks has a coinbase address where the miner fee was deposited; each address encodes a public key.

Write a message like “I am Satoshi. Here’s a boring detail which proves this message was created on demand.”, sign with corresponding private key, post the ~10 lines of OpenSSL commands which verify that the signature matches. The end.

If he is Satoshi he could sign a message with the private half of a key known to have been used in the past by him.

Details of public/private keys, hashes, etc are important, but at the core the idea is simple.

There's a large number of early coins that are publicly known to belong to Satoshi, all he'd have to do is move a few of them around.