It depends on the machine, and the game. Some games have a progressive jackpot that build up as people play it. And when they are up to their limit they are ready to pay one, or all of them. One example is the game called Tawa, I play it at a local bar, and the Princess jackpot on a 40 cent bet is up to $145, and it has been at $145 for at least a month. That means it could hit at any time, because it is at the limit and not adding to the payout any more.
Another game is called quick pay jackpots, it has a yellow and a blue jackpot, when they are both over 10, one of both of them are usually ready to hit, usually just one of them. I was playing it the other day and the yellow bonus was on 17 and the blue was on 7, I got the 3 bonus boxes and hit the 17 spins, and it paid $150 on a 50 cent bet.
Most of the other games do not give you any clue when they are going to hit a bonus, a few others do, but not many. You just have to play the games to figure out if there is a way to tell when they are ready to hit or not. But in general, most games do not show you any clues as to when they are ready to hit a bonus or a jackpot.