To help webmasters diagnose these problems, Google have a new Instant Preview tool in the Labs section of Webmaster Tools (in English only for now).

Here, you can input the URL of any page on your site. Google will then fetch the page from your site and try to render it both as it would display in Chrome and through Google Instant Preview renderer. Please keep in mind that both of these renders are done using a recent build of Webkit which does not include plugins such as Flash or Silverlight, so it’s important to consider the value of providing alternative content for these situations. Alternative content can be helpful to search engines, and visitors to your site without the plugin would benefit as well.

Below the renders, you’ll also see automated feedback on problems Google system can detect such as missing or roboted resources. And, in the future, Google plan to add more informative and timely feedback to help improve your Instant Previews!