Ok guys, I solved this one.
1. Create a new Playlist titled 'All Music' and drag all the music from your main library into it.
2. Sync your iPod/iPhone.
3. Click the iPod/iPhone icon, Click and Highlight the Playlist: 'All Music' that was just copied over. Go to the Menu 'File->Library->Export Playlist'.
Save the File somewhere easy to access.
4. Click back to your Main Library. Delete the Playlist: 'All Music'. Go to the Menu 'File->Library->Import Playlist'. Choose the File that you just saved.
The Playlist should return but it only contains the songs that transferred over to your iPod/iPhone.
5. Create a new Smart Playlist. Set the rule to Playlist - Is Not - All Music. Click the little plus sign ➕. Set the second rule to Playlist - Is - Music.
Click the OK button. BOOM, there's all the songs that aren't copying over to your device.
Good Luck.