Play in a casino that is licensed by the UK Gambling Commission, by law, any casino that has a license with them must reveal their RTP, must agree to the game being tested by an independent body and must display the RTP rates to its players. Pretty simple!
https://www.gamblingcommission.gov.uk/licensees-and-businesses/guide/gaming-machine-categories
https://www.gamblingcommission.gov.uk/standards/remote-gambling-and-software-technical-standards

Play with an unlicensed casino and you're going to have to trust their numbers!