You can monitor your site traffic during the ad campaign and see if it goes up by a reasonable rate per impression though?
Sure I suppose they could lie and you happen to have such a well crafted ad that it has a super high click rate and you are getting scammed but I think that is highly unlikely. Especially depending on your target it would be easy to check.
We wish they were that cool, the inventors of the modern mile were more concerned about land measurements. A square mile is 640 acres. Which neatly can be cut into quarters 3 times. 160, 40, 10.