The "Where Used" feature lets you easily navigate all around Composer and Posts to find out exactly where and how often a particular resource is being used.
Click on the "Where Used" icon in the resource details screen to bring up the Where Used window for that resource.
The window lists Banners, Boards, Domains, Galleries, Message Templates, Messages, Pages, Posts, and Shared Elements with a number after each location. Expand the accordion for any location with a number (greater than zero) after it to view the name of the page, banner, etc., where the resource is being used. Click on the name to go directly to that location and see the resource in use. Click on "Back to Resource" to return to the resource details in Resources.
All of this action happens in your Composer screen behind the Where Used window, and the window stays put! You can minimize the window by clicking on the arrow icon in the top-right corner, and come back to it when you need it again. You'll see a confirmation message before you close the Where Used window.
Another great feature of the Where Used menu is the "Removed from Draft" label, visible in the example above. This lets you know that the resource in question is no longer in Composer, but still exists on the public website. That change just hasn't been published yet.