I've compiled a list of "common" issues that 920 owners have experienced (including me)
1. Abnormal battery drain: If you don't have any background tasks running, and you're battery is draining at 1-2% per hour on idle, it's a bad phone. My previous phone would drain about 3% per hour with only "weather" running. My replacement is at 68% after 35 hours...and that's with usage.
2. Loose Unibody Design: If you press down on the top or bottom, that part will press in and create a "pop" sound.
3. Questionable Vibration: This is hard to judge, because you need another Lumia. I recently got a replacement, and the rattle inside is definitely loose and does not sound no where near as good as my old one. I even did tests by holding them both in my hand, and having an email sent. Definitely different vibration sounds. On the latest update, it's supposed to sound more "tame" and "normal" like any other phone. It won't do anything to performance, but if you're a bit OCD about these things...well...yeah.
4. Illumination: People have already mentioned that from previous posts. Just check on the bottom end.
5. Dust on front lens: You can probably see it or test it out.
6. Loose battery: If you move the phone side to side, you may hear a small "thud" on the bottom end of the phone. Common problem with plenty of phones. Does not do anything to performance though. Hopefully he has a battery app installed so you can see a chart of how it discharges throughout the day.
That should be it. A lot of stuff to look out for. The battery will probably be the hardest thing to test unfortunately.