The actual casino games are very rarely tampered with. Most online casinos end up purchasing casino software from one of a few different casino software providers. This includes RTG, Betsoft Gaming, Wager Gaming, Slotland, 21GNET, Amigotechs, Parlay and Top Game for casinos that accept USA gamblers. Non-USA casino providers include Microgaming, Playtech, Net Entertainment, Party Gaming, Cryptologic, WagerWorks, Random Logic and Rival Gaming. By in large, these casino software providers will make sure that the integrity of the casino games cannot be tampered with to make a player lose on a large bet or to have anything but a random number generator that determines wins and losses.
However, some casino software providers (specifically RTG or Real Time Gaming) will allow online casinos to adjust certain things. This includes the payout percentage on certain games as well as some of the payouts on the games themselves. The worst payout percentage for any game should be higher than 90% even for the jackpot slot machines. Any table game should definitely be higher than 95%. The best that any game should ever payout will be close to 99%. No matter what, the casino will have the house advantage over any casino game.
The one game that you should definitely watch out for is Video Poker. Online casinos will routinely lower the payouts for receiving hands like a Flush, Full House or 4 of a Kind. If you are playing the Jacks or Better game, these payouts should be 6x for a Flush, 9x for a Full House and 25x for a 4 of a Kind. I have seen online casinos that offer 5x, 8x and 20x payouts respectively.
Now, this is basically cheating the player because the payout percentage will drop considerably because of this tactic and most people won’t know it. However, this is also something that most casinos also did back in the mid to late 90s. The Flush used to payout 9x and the Full House used to pay out 15x until people made books about how you can beat the Video Poker machines.