In iTunes on the left there is a triangle next to the name of your ipod. Click the triangle.
While this method does show you what's on your iPod, I find it easier to just make a playlist on iTunes of all the songs you want on your iPod, and sync this playlist with the iPod. Manipulating songs on the playlist is, IMO, much easier than dragging songs in and out of the iPod every time you want to swap songs. Plus, this way, if you ever have to restore your iPod, you have a record in iTunes of which songs you had on it, and can have them back on your iPod with a single click.