To cooperate with the gambling jurisdictions of a country, every casino will need to apply for a licence. There are usually remote operators, but independent casino tester bodies like eCOGRA calculate payout percentages for games and feeds this information back to assess whether a casino is complying to the necessary standards.
To find out whether a casino is licensed, you will be able to find this on a casino site’s web page. A site will display its credentials on the footer of a page where there will be a stamp and it will also indicate where the website is registered. The UK Gambling Commission also has a search page where you can find the details of all licensed casino sites.