It's very easy.
The first generation has an essentially flat but chromed metal back, and the bottom is a dark grey colour around the dock connector and headphone jack. Second-generation models have a tapered back (that is, it's a subtle curve on the entire back), and the chrome covers the bottom.
More importantly: the second-generation has a rocker switch on the left side; that's the volume.
Your model number indicates it's a second-generation version.
Also, did you try, y'know, visiting Apple's website to see if it looks familiar?