First thing is to make sure it fits. If it is too big or too small you'll regret it. Beyond that, just look and see if the bike was cared for, whether it was used a lot or a little. Tire wear is an indicator that it was used, but that's not necessarily a bad thing as long as it was cared for. If it is scratched or banged up then start getting suspicious that this may not be such a great deal after all. A few scratches are OK but not if they are all over.