Best Fraud: This is a toss-up... There was a slot machine with a glass panel deck where all the buttons were located (including "cashout"). Someone figured out if you opened up the "help" menu on the screen (very small & in the corner), guests could put their money in but not cashout (help menu blocks cashout function & betting functions). The guest who's money was stuck would then leave their machine to find a worker who could fix it. Meanwhile, the offender would be a few banks of machines away, watching this guest until they could swoop in, undetected, and grt their money super quick by just exiting out of the help menu. It's a lot of work, but it went on for about a week before we noticed the pattern.
Suspecting Fraud: Demeanor. ALL demeanor. I can spot a possible slot cruiser from the moment they walk in, and about 3/4 times I would be correct. They walk around with rubbernecks, looking down towards cash amounts or button panels (that light up when cash is inside) instead of looking at game titles up higher. Also, who you associate with will tip us off. Known troublemakers will stick around their kind, so if you are mingling with them, I have my eye on you until you show actual gambling behavior. Tables are harder to pin cheaters, but when looking for card counters in blackjack you look for the obvious. Fluctuating bet amounts, not playing the bonuses (doesnt fit well with BJ strategy), etc.
Most Common: Ticket theft & Check fraud
Violence on "Fraudster": Unlike the movies, there are no concrete boots or bats to the knee. But, if it's a Tribal casino, they reserve the right to detain you however possible. I have seen Wanted people get spotted by police, and eventually get their face slammed into the machine as they are arrested.