Click fraud occurs in a number of ways:
1. A rival decides to have fun clicking your links in order to drive your costs up
2. a Web site that does context ads for one of the search engines, and who therefore gets paid when a visitor to their site clicks on your ad, generates fake clicks to earn a few bucks extra.
Google and other SE's perform ongoing analysis in order to catch it, and when they find patterns of click fraud, they automatically deduct the wrong charges from your account. In addition, if you think you have found evidence of click fraud in your reports, you can complain and they'll check for you.
Click fraud is an annoyance, but for most tourism advertisers it's a fairly minor one. You can find out more by searching the Web for articles on click fraud, there are thousands. You'll also find numerous vendors ready to sell you services to analyze your log files if you feel you are being victimized by click fraud.
More useful to most beginning advertisers is this warning:
Resist another kind of fraud altogether: people who claim they can "get your site into the top 10!" of search-engine results, or conjure any other dramatic results for you, for a fee. You might get this in the form of phone calls from salespeople. Hang up on them. Nobody can really sell you position on the main search engine results pages.
There is no real short cut!