Yeah, this is a pain point for sure. I think Susan is right, moving them to an empty shell will help you see them compared to one another, but the "Find a rubric" tool in the course still pulls from any existing course you may have, including the rubrics.
If you want to delete old ones, I would open the "Find a rubric" tool and just go course by course down the list rather than searching blindly through each class.
Another method might be to have someone in your IT department help with a script using the Canvas API to pull a list of rubrics for the user into a spreadsheet. It wouldn't be hard to write and it would allow you to work through the list directly.