Who knows which machines machines are winners and which are though on the player? The change person knows. He/she pays out winners and gives change to losers all day long. So when you are getting change to play the machine yourself, give the change person a $1 tip for helping you, and then ask if he/she can help you further by selecting a "good" machine to play. Remember to top again if you hit a good jackpot after following his/her suggestions.