This one is pretty straightforward—just hold the bike up, spin the wheel and look at its profile from the front. If there's any shimmy in the rolling pattern, you'll need to take the wheel into the bike shop to be trued. While technically not a big deal, it adds additional cost to the bike and may imply that the previous owner put the bike through a lot of stress. Also, be wary of buying a bike with used carbon wheels. Just like the frame, make sure to check for any cracks or structural issues on the rim itself or excessive wear on the braking surface.