Unless you click on "Automatic Update" it simple copy-pastes in the resuable-content in a new page and then forgets about it.
If you do have Automatic Update on the other hand, the pages instead store references to the resuable content (not the content itself) and fetches the reusable content on render:
The "storage format" of the content in a Publishing HTML field uses a special
<div> element as a "header" and corresponding
<span> elements to designate the placeholders where reusable content is to be inserted (note that I'm only talking about reusable content that is specified to automatically update).
The "view format" of the content is generated by reading the "header" and subsequently replacing the corresponding placeholders that follow. Hence I tend to refer to this as the "expanded" HTML. In most cases, you can get the expanded HTML using out-of-the-box SharePoint functionality.
And here is a mockup of how the HTML looks in a page using two sections of reusable content):
Here is some reusable content...</p>
...and here is some more:</p>
(Source of above and more details here)
This leads me to believe, that if you remove a reusable content it would just not render in the pages anymore leaving no need for finding the pages that uses it.