It's though.Google has two points going for them that is hard to beat:
1) A very good inventory of ads
2) A very good targeting system.
The combination makes for an unbeatable system. If you browse the web you'll find for example that a certain competitor to google constantly shows ads for Vonage on pages that haven't got a flipping thing to do with voip, long distance or anything related to Vonage.
I think that is a complete disservice to the webmasters (and Vonage, unless Vonage just wants their name out there). If the website in question is about real estate -- show real estate ads, not telephone ads.
To answer your question, specifically, when you are browsing the web, notice what web sites have ads that are directly related to the topic of the website and which ones have clueless ads.
Then you know where to start your research.