Simple Way to Notice Click Fraud

We constantly analyse our traffic. It's how we discover where visitors are coming from and what campaigns are having the best results.
It also is a basic way to catch greedy and stupid click frauders.
Looking at your top referring domains. You may want to see the top 50-100 but depending on the amount of your traffic you want to look at the domains in relationship to other well known referrers.
We recently uncovered a bunch of second-hand search engines that use Yahoo, Enhance and FindWhat for 1,000s of clicks. Tough to belive when some of our best referrers are getting beaten by these unheard of sites.
There is also a player who uses parked domains and domains for sale (so when people type in the domain they are presented with a page with 2 sentences about the domain and then 20+ search results.
There is no way we received 521 visitors from people who type in a domain that these guys were selling!!!
I am contacting the various engines and will post when each replies... tick, tick - let's see who has the best response time and answer.