I imagine there are digital fingerprints that the league recognizes and the sportsbook recognizes. A smart guy like BRick likely understands it and could explain it better.

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Be well

GO BEARS!!!

This is only a guess but here are the major pieces of information that they will easily have.

Location data for every bet down to a mile or less.
Betting trends.
Email/phone numbers/bank account info.

Eventually, you get a fairly substantial profile on how you are betting and more importantly how often you win those bets. You may just always bet on the Bears to win because you love the Bears. You may always bet on the Packers to lose because you love the Bears. You may live close to Halas Hall or in another spot where players may live. Eventually, it triggers something where they automatically search for your information. It turns out you are facebook friends with a few players, or you went to high school with one, or any other flags that would indicate you may know someone. Now, there will be a lot of false positives there but that is when a human jumps in and looks at who you gave tickets to, or they even investigate who you are hanging out with in public and other information with a private investigator. Eventually, they get enough and they get called to the principals office and have no real choice but to admit they did it.

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You do not talk to me like that! I work too hard to deal with this stuff! I work too hard! I'm an important member of the CSFMB! I drive a Dodge Stratus!

I could see that if this guy was betting any kind of substantial money..hecwas betting less than most of us.

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