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?
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)
How do you determine whether your budget was wasted on click fraud or just ineffective landing pages?
Surely click fraud occurs, but how can you tell?