In a way when casinos start they have bots and even players who test the platform, some even in god mode. It is a very valid question, but there are ways to find out, perhaps the easiest is reputation, some casinos have it because they have been verified and are handled on certain industry standards.
Among other things that help to have confidence in a (certain) casino is that these prizes, although they may seem high, most are so profitable that it would be stupid for a so-called serious casino to be involved in such a scam.
Perhaps it is repetitive in mentioning it, but the answer is as classic as your question about the OP, only use trusted casinos.
Only the viable solution for you to take on because there's no other way on verifying out things to be true other than to be an employee on the said gambling site or company and having those information.
Other than that, you cant really find any proofs that they've been cheating.I dont exclude out those reputable ones because i do believe that they are still making out a bit of those kind of shady acts.
This is just in my hunch though but not totally been proven out.
Sticking out with reputable ones is a bit better but doesnt guaranteed out fairness.Just play on where the masses been hanging out and dont mind if those competitions are
fair enough for you to believe on.