In a blog post detailing its ad quality efforts, Google gives some insight into what these individuals do: They "review web pages, test our partners’ downloadable software, and prevent ads from showing on sites that violate our policies. Depending on the severity and persistence of the offense, they may stop ad serving on that page or site, or across the publisher’s entire account."
Of course, human review is only one method employed by Google – and other ad exchange operators – to combat impression fraud. Automated tools also play a big role, allowing Google to monitor clicks and impressions for suspicious activity. These scanning tools will improve over time as Google implements machine learning that can detect bad practices.
Google says its efforts are paying off. In 2012 it identified 17% fewer "bad actors" than it did in 2011; this happened during a period of greater enforcement, suggesting either that the number of parties attempting to exploit real-time bidding had declined (unlikely), or that Google's rigorous approach has driven RTB manipulators to seek out less vigilantly guarded auction environments.