Why did Al think that the third slot machine was ready to payoff in Chapter 15 of Grapes of Wrath?
From his position manning the grill in the kitchen, Al keeps an eye on the number of people who use the slot machines. On "a paper pinned to the wall over the griddle", he records the usage in "three lines of marks in columns on the paper". Al knows the frequency in which the machines allow a winner, and from referring to his paper, realizes that slot machine number three, which "gets more play'n the others", is "ready to pay off". Taking a handful of coins from the register, he goes over and plays the machine himself, and on his fifth try, sure enough, "the three bars (come) up and the jack pot dump(s) out into the cup". Al scoops up his winnings and returns them to the drawer in the register, minimizing the loss from the slot machine to the diner.
Money, and the way it changes hands, is a central theme of this Chapter, which is an interlude in the story meant to give the reader a sense of the larger environment in which the events take place. Apparently Al's crude system of predicting the machines' behavior and depriving customers of their rightful chance of winning is an integral part of the choreography of give and take in which money is distributed in society at large (Chapter 15).