There are licensing bodies whose sole purpose is to serve as industry watchdogs, awarding operational licenses to those casinos that are legit, audited, and whose numbers and outcomes are authenticated. It is very hard to acquire an online casino license.
Legitimate online casinos seek offshore gaming licenses. The most reputable licensing jurisdictions include Malta, Gibraltar, Alderney, Curacao, Kahnawake, and Antigua & Barbuda.
There are also independent auditing firms such as PricewaterhouseCoopers that audit casinos on a regular basis to ensure their games are installed with random number generators so that the results of all games are fair and random.