Confluence Cloud and Confluence Server from 6.x added a plugins usage screen at (replace YOURSITEURL with the sub domain of your Confluence instance). Click the entry (which shows the number of diagrams) to obtain a search generated list of pages with diagrams on.

For Confluence Server 5.x (more recent CQL versions), baseURL/dosearchsite.action?cql=macro+%3D+"drawio" should also work. (Replace baseURL with the base URL of your Confluence instance).