Google now offers basic statistics for its sitemap service. Sitemaps was introduced several months back as “an experiment in web crawling.” Putting sitemaps in websites helps the Google crawler index your webpages.
Google now allows you to check, through sitemaps, whether your sites have pages that cannot be crawled by the search engine. If you have a verified sitemap, you can view:
 Sitemap details and errors
 Indexing information about your site
 Query stats about your site
 Crawl stats about your site
 Page analysis of your site
 URLs from your site Google was unable to crawl, and why it couldn’t crawl them
Verification is very easy, you just have to upload an empty HTML file with a unique filename. Sign up for Google Sitemaps using your Google account, by clicking on this link.