Never. Never trust a gambling site or any other website for your credit card. Only trust Paypal. There are alternative for Paypal but you don't have to use them as long as you're not having any problems with Paypal. To know that a gambling site is not a scam, just use Google. Google is always handy on finding out the answer for your question. This question of yours can even be searched on Google and you would find good results. It also saves you a lot of time.

There are websites out there where the owner has collaborations with other people and they keep on updating their website on the new gambling sites. They then investigate that gambling site and see how legit it actually is. One way on finding out is to use the website by themselves. Another one is by looking for existing feedback about the gambling site.

tl;dr: Use google.