well personally I would not bother with it especially if your new at bike maintenance and repair. if AEO is correct about the failure rate I would consider it a waste to buy a used one.

as to what to look for... first of all make sure the wheel is true (doesn't wave side to side when you spin it), look for flat spots, (take the tire off and spin the wheel if it dips it may have a flat area), the bearing feel smooth with no play or gritty feel, and look very closely where the nipples and spokes enter the rim, for cranks bulges. wipe it with a rag and use a flashlight.