Dang! I don't see an
easy way
You could create a 'test' Library
Then Import all books to that.
Then with your main library filtered by YOUR (saved) 'Known on the Device' Tag, do a duplicate check between libraries :find Library duplicates: Invert the results
You could preset (in preferences: adding Books) a tag: 'device only'
then add the books to the Library and do the resolve/cleanup of those tagged