I used to work in a bank and the paper was usually the tell that the money was funny. If you pass one bad bill it may pass, it is when you have one bad one in a stack that makes it stick out like a sore thumb. That is how I found the few that people tried to pass on me. Of all the funny money I found, it was a poor businessman who took it in not a counterfeiter. I would think that most of the counterfeits that casinos take in would be from people who did not know it was bad. If I were a counterfeiter I would try to pass them off in a busy, dark place like a bar than in a casino or bank with trained people looking for them. That said there are a lot of stupid people in the world. In the time I worked for the bank I never saw a bad $100 they were all $20's. People tend to look at the $100's closer than $20's but that was some time ago and that may have changed.