If Google was to announce exactly which sites your ads were being displayed on, you could effectively go straight to those sites you thought were giving you the best ROI, and do your advertising business direct. Which also means that Google wouldn't get paid when you do business direct.
You can try surfing through sites that you think would be your target audience and see if your ads are being displayed. And don't forget they could also be displayed on one page view, but not the next, so it is not foolproof.