Most the time by user experience.
Just play on some big casino or old, you should fine.
Good idea, mate but some competitors use this "SCAM" word + casino name scheme to beat their rivals. Currently, almost all casinos are "SCAMS" as you will definitely find a website, where they are called scams, even decent ones:) When I deal with fiat casinos or even with some crypto ones, I personally look at license (Curacao) and their reputation on Askgamblers or Btalk. I know that casinos tend to pump their reputation by buying reviews, but if you read some 10-30 reviews, you will find some true information about a casino. All in all, gambling is more about fun. Even if you win a decent amount, you will not be able to withdraw it all due to withdrawal limits that you can see in all casinos be it an excellent casino or a notorious scam
If the accuser use a newbie account then the accusation would not count so I guess its good for people to not take their allegation seriously since spitting some dirt to other business exist so always look for valid feedback from real user.
But any of this the easiest way to find out the best casino to gamble on is what @ryzaadit say the old one is the best since many tried them and they are running good so far, we can find many old running successful casino in this forum and they are best gambling site to choose.