Every BC Originals round is generated from three inputs: a server seed we commit to before you bet, a client seed you can change at any time, and a nonce that increases with each bet.
The hash of the server seed is shown before the round, so the outcome cannot be changed afterwards. Once you rotate your seed pair, the previous server seed is revealed and you can verify every past result.
Open any bet from your history, copy the server seed, client seed and nonce into the verifier of that game, and compare the computed result with the one you were paid on.