75-ball bingo cards. Players use cards that feature five columns of five squares each, with every square containing a number (except the middle square, which is designated a "FREE" space). The columns are labeled "B" (numbers 1 15), "I" (numbers 16 30), "N" (numbers 31 45), "G" (numbers 46 60), and "O" (numbers 61 75).
You can spot them pretty clearly as a bingo card is placed on the screen and the games are rendered against the cards as the slot reels are spinning. Some players have played them enough to be able to spot winning/losing patterns on the cards faster than the reels can render.