How do I find out which audio files won't transfer to iPod

I have more files in my music library than in my iPod. At a quick glance through the library All files look compatible, yet some won't transfer. Searching through 20,000 songs to find 5 is a tedious process.

When iTunes can't find a file to transfer it pops up a message, but there's no message when it can't transfer a file that does exist. Does anyone know if there's a log file or some hidden place to look to see which files won't transfer so I can fix them?