Search Console now shows the estimated severity of blocked resources with the Fetch & Render tool. Embedded content (like images, scripts, CSS files, etc) sometimes plays a big role in how Google's able to render & index a page for search.
The Blocked Resources report ( http://googlewebmastercentral.blogspot.com/2015/03/unblocking-resources-with-webmaster.html
) gives you a bigger picture of the embedded content that's blocked (on your site, or on other people's sites -- which can be just as critical!), and it's a great way to start drilling down into this issue.
To check an individual page, you can use the Fetch and Render tool ( https://support.google.com/webmasters/answer/6066468
) to see how Googlebot is able to view a page, and compare it to how a user might see the page. We've extended this tool to give you a rough idea of the severity, approximately how much of a page is affected.
Why is this an issue at all, why can't Google be happy with just HTML anymore? For one, if your site is mobile-friendly, we'd like to recognize that. Also, some sites use embedded content to pull in additional information that we'd love to show that site for. For example, if you're using an API to pull in and display your business' branches + opening hours, we'd love to use that to send people your way... but we can only do that if we can see that content.