Music Library Magic analyzes your iTunes library and the music files you have on disk to help you keep the two in sync and without duplicates.

Once you drag in the library and the disk folders it will show you what files appear more than once in your library or on disk, what files are in your library but not on disk and what files are on disk but not in your library.

It will also show you what files have changed on disk without iTunes being aware of those modifications and lists them under "Changed files on disk out of sync"