How do you know if the gambling site is a scam or not? how you trust a gambling site for you credit card and know its not a scam?
its hard to predict or guessing,scammer have their own time to decide when they going to scam,and its why we can't identify which gambling site would be scam,for me,i just play short time,not keep my assets on gambling site for long timeand of course i try to patience if my witdrawal suddenly never come to my wallet 
I think research is the only way to find out about scam gambling websites . You can read reviews about the website and test them with little amounts of bets employing short time plays and I think you'll be able to make an informed decision with time.
A quick googling of the gambling site's name would yield all the results, good and bad. So that's the first step and then search them here on the forums if you are looking at a bitcoin gambling site. I believe their are sites online that review gambling sites whether they are a scam or not and puts honest reviews about them. But the best thing to do for every new gambling site is to do test deposits.