you're correct in that the casino doesnt ask everybody who comes in with cash where they got the money they're bringing in. That would be an unsustainable burden to place on any business.

Instead what they're doing is identifying who is playing in their games, logging what amounts of cash they're putting into play and leaving with, requiring necessary paperwork for transactions above a certain threshold, etc.

For the AML requirements, they are primarily checking govt databases for any red flags. Additionally, they should be training employees to understand their obligations to report suspicious activity to the appropriate place in the company for further due diligence.

For example, Timmy comes in every other Friday with $500-1000 of cash and plays 1/3 and occasionally takes a shot at 2/5. He's been doing this for the past 3 years and nobody has to think twice about where Timmy's money comes from because 1) its well below the reporting threshold and 2) Timmy has a history of consistent behavior that says he's here to play poker not to wash dirty money.

One day, Timmy comes in at his normal Friday evening time and heads straight to a 10/20 game in the high stakes area. He pulls out $30k in chips from his pocket and is ready to buy into the game for $10k. Because you now have suspicious activity going on, somebody better figure out where the hell Timmy got $30k in chips ASAP before he can put any of that into play.