You have to work through each function that an add-on provides and look at the config or logs, or activity.
Add-ons can do so many different things, there's no way to know where they all might be being used, and the methods to work out their usage vary wildly.
If, for example, an add-on creates a custom field type, it's easy - run a search for "custom field of that type is not empty". But if it provides a report, you're going to have to read access logs to see if people are using it. If it's a workflow function, you need to read all your workflows (although a search for the plugin key in an xml backup is usually easier)
The most reliable, but potentially evil method is simple though. Disable the add-on. See who screams.
You're still going to have the same problem that add-ons can do all sorts of things, but macro usage is solve-able.
I have https://community.atlassian.com/t5/Confluence-questions/Scan-Confluence-instance-for-macro-usage/qaq-p/117041 in my bookmarks - Davin's macro works really well.
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