However, Apple moved to random serial numbers around 2010+, so this only works for iPod Classic, Mini, 1st‑gen Nano, and Shuffle.
The location of the serial number varies depending on the specific iPod model: check apple ipod serial number
Look on the inside edge of the clip (2nd Gen) or the back of the device under the clip (3rd Gen). iPod classic: Engraved on the back case toward the bottom. Using Software to Find the Serial Number However, Apple moved to random serial numbers around