[GoodIntentions]

analytics.

content on 3rd party site->ad->your landing page.

analyze what happens. You can compare normal behavior - time on page, number of pages viewed etc occurring from normal organic traffic against traffic generated by the ad campaign. Did one site or group of sites send 90% of your clicks, but almost zero real traffic?

[jfoster]

Another thing I've seen happen through Adwords was a large percentage of traffic coming through with default language set to Arabic and spending 0 time on site. (on a site intended for English speakers)

[proveanegative]

Couldn't the fraudster stimulate human browsing behavior as well, at least to some degree?