Only if you own both the site and the ad network otherwise you'll be unable to tell if they made a request for the ad or not. There are also ways of getting false positives with that sort of test, for instance if for whatever reason the server that has the ad on it isn't reachable from the client (say a problem in one of the major backbones, or a bad DNS entry).

The usual approach I've seen sites take is they have some sort of "Please disable your ad-blocker" image that they load everywhere ads go, and then just overlay the ad image on top of that one when it loads.