Nothing you can do really, besides checking for standard misuse on wires/wheel/pedals - visible inspection
All you can do is to make sure everything turns smooth, full 900 degree rotation. Make sure pedals travel properly. Make sure wires/cabels are not bend (fx. G25/27/29 pedals doesn't really have a gentle "entrace" for the wire into the pedal base). Push alle the buttons to see if any of them stick and so on.
IMO i would prefer to test a wheel before buying. At least put it into a PC and verify that everything is aligned properly (eg. deadzones and such). But if the guy seems legit and the wheel looks good, then shouldn't be too much of a fuzz