The adsense preview tool.
This is a natty little tool that you can download from google, at https://www.google.com/adsense/previewtool#2,
once installed on your machine, simply right click on any web page and
see what ads google would put on that page if you subscribed to their
adsense program. It's invaluable for understanding what google sees
as the theme of your site. You may be surprised and decide to do some
remedial optimisation work.