Confluence Cloud added a plugins usage screen at (replace YOURSITEURL with the sub domain of your Confluence instance). 

For Confluence Server 6.x navigate to baseUrl/admin/pluginusage.action. (Replace baseURL with the base URL of your Confluence instance).

Click the entry (which shows the number of diagrams) to obtain a search generated list of pages with diagrams on.

To list diagram attachments, baseUrl/label/drawio.