How to tell if somebody opened my ipod touch 2nd generation?

I sold an ipod touch on ebay and had it returned saying it was received broken. I heard of people returning broken ipods as a scam and so i jotted down the serial code on the back of my ipod touch before i shipped it. the code is the same but i know for a fact the ipod touch was working. whether it broke on the way there or i got scammed i just want to know: is there a way i can tell whether or not my ipod touch got opened? i mean i hear apple voids your warranty after you open your ipod touch but how do they know if somebody has?