The 'provably fairness' principle ensures you can randomize and verify fairness of your bet.
Before you bet, randomize it with some integer (known as 'client seed') that casino can't predict (can be generated as XOR of hash function result parts).
Before you bet, you can also obtain a hash of 'server seed' byte array.
So after you bet, the casino sends to you a bet result and the 'server seed' byte array itself.
Suggesting that the casino lets you know all the algorithms, now you can verify your bet using 'client seed' and 'server seed' numbers to calculate the bet result and 'server seed' hash by yourself and to compare them with casino's bet result and with the hash sent to you by casino before bet.
When results and 'server seed' hashes are matching with casino's ones, you can be sure you bet was proven as fair.