If you run AdWords ads on Google, you should turn on click destination URL auto-tagging - which appends a unique ID to the end of each click request from an AdWords ad.
Then after a month or so, you can look at your campaign report and see how many clicks Google considered "invalid". I.e. Click Fraud that they don't charge you for.
I just ran a report like this for the last month for a campaign with over 100k clicks in that timeframe, and Google counted roughly 3.5% of the clicks as invalid.
I didn't check that against my logs yet, but it should be do-able, and I could also use some of my own click fraud detection algorithms to see if I get the same result. I'll report back when I get a chance to do that.
I was thinking of putting up a site to let people report their Invalid Click rate anonymously, but I'm too busy with the shopping season right now!