So how do you know what is blocked? Well, Google have recently announced that they are including a report with a list of these blocked resources in their webmaster tools. The report will firstly show you where the blocked resources are being hosted, then on clicking on the host will show a list of the pages and resources that are blocked.
If you have any blocked resources go to your webmaster tools and use the fetch and render button (under Fetch as Google) on these pages to see how Google sees your pages.
If there are many resources that are blocked, Google recommend starting with the resources on the hosts that you own since you have access to these files. Begin with the resources that might affect the site’s content or layout first, once these have been fixed use the fetch and render tool to make sure Google can now see these blocked resources.
If you would like to know more about Google’s blocked resources report please click here.