They are truly supposed to be random. But it is said that some casinos program the machines with less probability of jackpots. So what people like you and me do is play at a casino which guarantees 100% fair game play.
Whether we are talking about physical or online slot machines, they all rely on Random Number Generator (RNG) technology to ensure fairness. The RNG is a sophisticated algorithm that generates random sequences of numbers, which determine the outcome of each spin of the reels on slot machines.