Microsoft has released a study that details how tracking an entire user session helps to explain the intent of searchers by looking at what they do before and after they search - and this will also reveal when a search engine click through is fraudulent. 

I reviewed the PDF study created by Microsoft Research and found this chart (below) very meaningful.

Much of the ability to know if behavior is click fraud is achieved by comparing the behavior through a session with average behavior. 

Here’s the PDF link, in case anyone is interested in reading the entire Microsoft Research Paper on Click Fraud.  I found this information via MarketingShift.