You can't. Any add-on will provide a set of functions that could be used anywhere. Some may be limited to projects, some are identifiable by saved data (a custom field will, well, exist on the list. A post-function will be referred to in workflows, etc)
But there's no way of knowing where something like the CLI connector might have been used - it can be used to do things that don't store any data you can even begin to search for.
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