More than likely, you have some repeat songs in your library. The only way to find out which songs are on your iPod and which ones aren't is have your library window right next to your iPod window (both through iTunes) and scroll through every song to see which ones are missing.
You could also scroll through your library on your computer to see if there are any repeat songs. You should arrange your library by song title. That way, you will have a easier time seeing repeats.
64GB iPhone 5 | 64GB iPad mini | AppleTV 2 (2012) | AppleTV 2 (2010) | 2012 15" MacBook Pro, 1TB SSHD, 16GB DDR3 1600 MHz, OS X 10.8.3 Mountain Lion | Apple Lossless | iTunes AAC 192kbps VBR | iTunes 11.0.2| Library size = 1.04TB | Legacy iPods: 3G 40GB, 4G 40GB, 5G 60GB, 160GB iPod classic (2009)