should be fairly easy to do - the original code is still on the Ethereum blockchain. just compile his code and see if the bytecode matches the original - even the slightest modification would make it fail the check. the only required knowledge are the compiler settings, but they're so few that you can brute-force them just to be sure that you're not falsely accusing him of having changed the code.
(btw, I've deliberately outlined a protocol that doesn't rely on other third-party resources - if you're happy to rely eg on Etherscan you can do a simple diff against the original code)
