I think there are reviews that you can search online or find in forums such as this, and that could aid you in verifying or validating. Reputation plays a big part - surely if a gambling site has a sketchy history, people would talk about it so better keep your ears open for that. So far, I don't think that there are any other regulating bodies which makes verification because of the conflict among jurisdictions. The best a customer could do is due diligence and research on the background of the gambling site or operator.
There are tons of gambling site reviews that you can google up to see for yourself the validity and fairness of each site, I am not pinning any link because you can research on it yourself, And because gambling site can surely come and go you will sure base only with reviews and having a trustful thinking with your favorite site that it is truly legit, Well their reputation is on the line so you should see that consider reporting gambling sites that have scam accusation and problem with their fairness to be reported in the forum so others can see and be aware of it,
Well I don't know if there are regulating bodies that are keeping tabs on a gambling sites activities and recording their every move, There are tons of review about a certain gambling sites but you can not call them regulating bodies, I would like to call them volunteer bodies, Because they are doing it for the love and support and not for something else.