Google have also added another nifty little function, the Site Health report. If Google hasn’t found anything to worry about it will display the text “nothing to report” next to the thumbnails of your sites.
If though it has detected something you need to address it will display a Site Health alert icon
At the moment the things that will trigger the alert are :
Google detects malaware on your site
An important page is removed from your site
An important page is blocked by robots.txt
Another good reason why you should be using Google Webmaster Tools.
You might also like to read :
Site Configuration - read what the site configuration section covers.
Your Site on the Web - read about the Your Site on the Web section.
Diagnostics - Read what’s in the diagnostic section.
Labs - Google playground.