If the casino does not directly show the rtp of the slot game, how can we know the RTP version of it?
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-categorieshttps://www.gamblingcommission.gov.uk/standards/remote-gambling-and-software-technical-standardsPlay with an unlicensed casino and you're going to have to trust their numbers!
How can we win in the long run if the RTP is always lower than 100%?
You can win even if the RTP is 0.1%, that number is for all the players not just yourself, if 99 guys lose 10 dollars and one guy wins $500 he has won with an RTP of ~95%.