Check for screen damage, both the glass and the display itself (dead or stuck pixels, etc). Check battery performance. If the back is slightly rounded your battery may be bulging which is something you DON'T want to happen. Cameras: no scratches on the lenses, no weird behavior on the autofocus. I see it's a cellular model, so check the performance on that. Buttons like the Touch ID-sensor and the volume/sleep-wake switches should work. Some scratching on the back is normal. Check both the microphone and internal speakers as well as the headphone jack (both on audio out and microphone).
If all those things work out, you have a good 64 GB iPad Air 2 for that price.