Google Webmaster Tools is a valuable source of information if you have a website: Google lists crawl errors, popular searches that lead to your site, backlinks and SEO tips. Until now, Google listed the URLs from your site that returned the 404 (page not found) error, without mentioning how GoogleBot found those addresses. This has been fixed and you can find the list of pages that link to broken URLs from your site in the "Linked From" column of the "Diagnostics > Web crawl > Not found" report.
"If we report crawl errors for your website, you can use crawl error sources to quickly determine if the cause is from your site or someone else's. You'll have the information you need to contact them to get it fixed, and if needed, you can still put in place redirects on your own site to the appropriate URL,"
suggests Google Webmaster Central blog.