It'll not only tell if you if they have AdSense, but also what size ad blocks they use.
11:18 pm on Jan 31, 2010 (gmt 0)
Look at the source code on a site as well, it will give a publisher number in the code.
11:11 am on Feb 1, 2010 (gmt 0)
The short answer to the question is to simply look at the site. That is so obvious that there might be a hiddent sub-text in your question. Perhaps you should expand.
2:28 pm on Feb 1, 2010 (gmt 0)
In internet explorer, you can click on tools "View source code" and it will show you all the HTML for the page. Then it should say Google Adsense and a publisher number in a snippet of code where the ads are placed in the html.
1:11 am on Feb 18, 2010 (gmt 0)
There is a 'placement tool' in the 'Networks' tab of your ad groups. Click the 'add placements' button to view it. This will allow you to search for whether a specific URL uses AdSense, or all sites of a category, etc.