Online casinos ensure fair play and randomness in their games primarily through the use of Random Number Generators (RNGs), which are sophisticated algorithms that produce random outcomes for games like slots, roulette, and poker. These RNGs undergo extensive testing by independent third-party auditing firms to verify their randomness and fairness, ensuring that results are unpredictable and unbiased. Additionally, many reputable online casinos display their certification from these testing agencies, providing transparency to players.