Other than visually for obvious damage, lift the machine off the ground and try to move the wheels and feel for play. From the outside push against the top and pull against the bottom and vice versa to try to rock it in and out that way. And also grab the front and back of the tires and try to rock that way too. Hope that make sense. Bearings are sometimes harder to feel and have to be removed to spin by hand. Also spin the wheels and listen for any noise or grinding of the bearings.