I'd love to be able to look at the music in my library and see what's in a library and what's not.
When you look at the library, the songs you see are in your library.
Songs you do not see are not in your library
Perhaps you mean you'd love to look at the music in the library and see which playlists the songs are in?
You can select a song, then right click > Show in Playlist.
It will list all playlists this song is in. If you select one, it will open that playlist.