John Mueller of Google reshared a post on Google+ originally shared by Google's Ilya Grigorik with information on a tool that helps webmasters find and ultimately fix those hard to find mixed content issues on your site. You know, when you have a web page someone on your site that has something embedded using HTTP, on your HTTPS page, which results in a broken HTTPS/SSL lock at the top of your browser.
You can find the full array of tools over here.
Keep in mind, currently, having mixed content HTTPS errors doesn't impact your Google HTTPS ranking boost but it may one day.
Forum discussion at Google+.