One of the first things I always check when I look at a used bike is the spokes. I'll pluck my finger across all of them and listen to the tone, a well tuned wheel has them all pretty close to the same pitch. I've seen wheels on used bikes where one spoke was broken at the hub side, but it was still in place so you couldn't tell til you realized it was totally loose. spin the wheels and make sure there's no dents or tacos on the rim