Confluence 6.0 introduced a “Macro Usage” admin page for this. You’ll find it under BASE_URL/admin/pluginusage.action.
This page tells you if any user-installed macros are not being used. For those that are being used it shows you how many times they are being used:
Clicking on the link of each macro takes you to a Confluence search of each usage of that macro, so you can view individual items.