Does Google know what your page is about?

One of the things I discussed while walking in Cornwall was how can you tell whether Google knows what your site is really about.

There are a load of ways but one way we decided to try was to run the AdSense Preview tool over the site in question. If you see ads that are relevant then Google probably knows what your site is about because the algorithm that determines the content of your site is very likely to use a similar technology to the search engine. After all, why re-invent the wheel?