Investigators would not say what tipped them off to Bergen. But they have ways of spotting suspects. Gamblers who try to wager with slugs or shaved coins often hold two cups so they don't mix their winnings with the fakes they use to gamble. Then there are the yo-yo players, people who attach a string to a token and pull it out to use again after making a spin. The constant motion of slipping in a coin and pulling it out looks like they're twirling a yo-yo.
People have patterns.
People have patterns when they're playing. People have patterns when they deviate from normal patterns. Investigators can monitor the slot machines for payout abnormalities. If detected, they review surveillance footage during the time of the hit. Photos of suspected players are scanned into a system that can match faces to pictures of known criminals by measuring the distance between a suspect's eyes, nose and ears.