If you activates, using for example, in the John Chow page, you can see what people see in other countries.

You can see too the urls of the anounces and the apparience. It´s great, because you can try what people will see in other countries, usually the system shows the best paid I guess. Too is great to know some urls unknown to filter.

The usual features includes

Check the destination of ads that are on your page : ads displayed in the preview tool are in a test state, so you don’t have to worry about accidentally clicking on an ad. Clicks and impressions are not counted against the ads that display within the preview tool.

View sample ad formats and colors : create previews of formats and color combinations within the preview tool. Choose from 2 different preview formats, plus a 12 ad spread that allows you to view more available ads at once.

Preview colors on your page : the ‘Preview’ feature of the Color Options menu allows you to see what new color combinations would look like on existing AdSense standard ad units, right on the page. By simply selecting new border, background, or text colors from the preview tool, and clicking “Preview,” the new colors will be temporarily applied to the standard ad unit currently on your page.

Geo-targeted locations : select the geo-targeted location from which to view ads, allowing you to see what users in Canada will see, even if you live in Japan.