The jackpot is triggered by a specific combination of symbols on the reels. When you pull the lever or push the button, the random number generator generates a mixture of symbols. If the given combination matches the jackpot combination, you win big time.
No, there is no way to predict when a slot machine is going to hit a jackpot or to determine which machine is going to be lucky. Slot machines are programmed to use a random number generator (RNG) to determine the outcome of each spin, and the RNG generates random and unpredictable results.