This is a difficult question, as most the time you do not find out until it is to late. But one thing you can do is only join casinos on sites which you trust. As most people who put up banners or links to casinos have experiences with the casinos themselves and one thing for sure is that if a casino cheats a player they will definitely cheat a advertiser. Another thing to watch for is how long the casino has been in business.
If it is fairly new it may not mean the casino is not honest, but some casinos are fly by night operations. Some have very good intentions when they start up but do to operating cost and other expenses they just cant make it and fold. Now some may think that because a company is publicly listed it means they are honest. WRONG! Some of the publicly listed casinos are the worst. Cheat, scam, trick all kinds of tactics. You can notice some of these type when visiting a search engine and typing in a keyword to search for and when you find a site all of a sudden you are re-directed to another page. Nothing to do with the subject you typed about. They have tricked you into coming to there site. Now if they are willing to play those type games on your searches, what kind of games do you think they would do with your money when and if you win? Play at honest casinos, that don’t have to trick you to come to there webpage.