how to determine what songs are already sync'd to mp3 device (zen mozaic)?

I'm using songbird to sync music to my creative zen mozaic, and it works great. My only problem is that I have so much music its hard to know whats already been synced. If I just sync all my music, hoping only the new songs will transfer, it syncs all my music again and I get duplicates in my player. Is there a function or addon that shows what songs are already synced to the device?