Its not that easy to answer your question. There is no 100% accurate list with pointers on what to look for on an old iPod. There is
always a risk with old electronic devices, and even more so a device with moving parts.
If you can look at it in person, you could always check for dings and dents on corners and on the edges. This would suggest it has been dropped and that is never a good sign.
You could also listen to the sound from the harddrive. If it sound like an agricultural machine or if the harddrive reader-head seems to jump back and forth ten times before playing music, then I would suggest finding another iPod