There are some events that are so blatant that you can detect cheating, but they will tend to be low level games and possibly even involve both sides trying to play a particular way (e.g. very badly). Besides that it can be very difficult to definitively prove that cheating is taking place unless you're able to investigate the paper trail or catch the match fixers proposing it directly. The bookmakers will be able to see unusual betting behavior and possibly take action, but unless they have proof it is unlikely they'll do much except ban the person from placing any more bets or cap them at such a low level as to make it pointless. As it could effectively end someones career if detected or suspected, the players involved are going to want substantial sums upfront, which means the cheat has to pay off quite big to make it worthwhile.
Bookmakers, if they suspect the game of dishonesty, simply cancel bets on this event - they simply return the money. They don't have to prove anything, go to court, etc. these are the rules of the betting business, and by the way, they don't even have to watch the game to analyze it for fraud - if they see that an unusual number of bets are being placed on this event, this is enough justification for them to take action.
