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Came across a good thread over at Cre8asite forums about Google refunding some money to an advertiser using AdWords that received some fraudulant clicks from their content network. It was encouraging to see a user report this refund. Google states they are "very serious about catching invalid clicks", and use specific tools and techniques to catch on to fraudelent behavior. They do not however disclose the details of the filtering technology that help catch robots or users who click for inappropriate reasons.
The member stated he saw an increase in CTR from 3-4% to about 12% in a short amount of time. He suspected something but didn't contact Google about it. Google quickly responded to the situation, and he received a refund in his bank account for the fradulent click throughs. Ammon Johns, suggested one site to help monitor click fraud, at Click Detective. Seems to be pretty useful for catching this, they are not the only service however, so if interested its probably wise to investigate a couple other ones.
Read up on Google Click Quality Standards
