Have someone check the person wins, if they win hand them money or if they earn tokens through some system with VCE convert those tokens to money for the player. That's what I think Icygamma and the others are basing the casino's/betting systems ideas on.
That seems a bit silly, I understand the concept of strict roleplay and the honesty system, but wouldn't it be better to have some sort of specific casino event that couldn't be tampered with so it's all automatic? I wouldn't want to play for example, a slot game, and win only to have someone play and change the results while I'm asking them to be validated, for instance.
If I were you guys I'd make some new event for casinos that would allow you to pick from an odds level (low odds of winning, medium, high - this prevents the casino owner from doing some ridiculous 1 in 1000000 chance of winning) and then maybe have a parameter for how much the person has to bet or something. Obviously I just made this up on the spot so there's tons of room for improvement, but it's only a suggestion.
Of course I also have no idea how the current casino works. Could someone enlighten me on how winnings are dealt there?