Google has released a new feature within Google Webmaster Tools, called Index Status, that allows site owners to see exactly how many pages on their site are indexed and when.

The Index Status tool is found under 'Health' and displays four metrics, as Google explains:

• Ever crawled: The cumulative total of URLs on your site that Google has ever crawled. Not all crawled URLs get indexed, and Google may discover some URLs by other means such as inbound links from other sites. This number should increase over time as new pages are added to your site.
• Total indexed: The total number of URLs currently in Google's index. These URLs are available to appear in search results, along with other URLs Google may discover by other means. This number will change over time, as new pages are added and indexed, and old pages are removed. The number of indexed URLs is almost always significantly smaller than the number of crawled URLs, because it does not include URLs that have been identified as duplicates or non-canonical, or less useful, or that contain a meta noindex tag.
• Blocked by robots: The total number of URLs that Google cannot crawl because they are disallowed in your robots.txt file.
• Not selected: Pages that are not indexed because they are substantially similar to other pages, or that have been redirected to another URL. More information.

Read more about the Index Status tool on the Google Webmaster Central Blog.