If you click on the Google Index tab on the side bar of GWT, you’ll see a link for Index Status.
The Index Status provides stats of the URLs that were indexed for the site in the past year. Here you will find the total indexed, blocked by robots and removed. There is also a line graph below. A steady rise for total indexed is good. If you see that there is a sudden drop in indexed pages, it might mean that Google is having trouble accessing your site.