The most common way players cheat at roulette is by tampering with the roulette wheel with useful roulette odds. Any bias on the wheel can cause the ball to land more often in certain spaces over others. The wheel can either be tilted slightly, causing the ball to favor a certain side of the wheel, or individual spaces can be rigged, convincing the wheel to land in those particular slots on a regular basis. Players who have rigged the wheel will then place their bets on those spaces and will cheat the casino and other players out of their money.
Other players may cheat at roulette by placing or changing their bets to reflect the results of the game with effective roulette betting. While this can't be accomplished in front of observant dealers, some players will wait until the ball has already landed in a spot. When they see where the ball is, they will nudge their bet over to that space and claim that they had always placed that bet. Others will wait to place their bets until after the wheel is already spinning, giving them an unfair advantage over players who placed their bets legally.
By watching for players who cheat at roulette, players can protect themselves from losing out and having their own bets nullified.