Most of the casino works with big money, So they won’t scam for the little money. Which you mean as the bonus to the participants, the bonus is nothing to the trusted gambling or casino. If you want to check to find the bonus program of certain website, you should apply for it. Then check whether they paying you the bonus or not. The good gambling site ready to pay you even 1000 dollars as a bonus for the big deposit. Bonus program is used by the website to get attractions among gamblers, I don’t think they will scam on such program.
Hmmm maybe it is still possible for a casino to say that they give this amount of drops in this xx people but then OP can try to check for the website traffic, I am not so sure but maybe semrush can help? it can help you to know how many visits per day does this website can have maybe with some deep searching he can also see if they sent a lot of email to the users? again only the traffic.
The promotion model (prize draw for limited number of users) at online platforms involving thousands or hundreds of thousands participants from different places in the world can't work in a 100% transparent format, because there will never be a certain way to make sure every winners are real legit players. I believe website traffic neither any other tools can help on this case. That is the kind of competition you have to trust a lot the house's legitimacy to participate.