You use whatever you can use to check the validity of the casino and the reputation that they have. Use the internet for searching, the forum for checking their trust and reputation, you have a lot of choices that you can use just to clarify the doubts that you have in your mind.
You can actually look for prospect casino sites, then perform an intensive research with regards to it.
I usually do that, I look for the site which is kind of good, then check for reviews, if it has documentation,
did somebody was able to withdraw and
so on.
This is what it should be. The research will come from you since you're the one that's studying the background of the casinos where you're about to gamble.