Something like this typed into google would be ideal, if I was bidding on the keywords "brown widgets":
------brown widgets pagead2.googlesyndication.com/foo.js
Of course, that does not actually work. Is there a way that does?
If you take user feedback, perhaps ask how they found your site - you might get some leads on where your ads are running if they say "oh, I saw it at xyz.com".
I doubt Google will offer advertisers more specific information, because that would give advertisers the information they need to go right to the sites with the best ROI, effectively cutting Google out of the picture. So while it would be nice, I doubt it is a feature they will ever offer.