I run Ghostery to stop sites from loading external media. If you're not opposed to plugins, even if you don't actually want to block the media, you can put it into disabled mode and it'll still tell you how many external calls were made (and, if you click on the icon, it brings up a list of the associated domains).
Granted, I'm running Chromium (presumably you're talking about Firefox), but the plugin does exist for both browsers.
Granted, I'm running Chromium (presumably you're talking about Firefox), but the plugin does exist for both browsers.