The obvious thing to check about both the front and rear wheels are that they are straight. Lift the rear wheel from the ground and spin it to ensure it is straight. At the same time make sure the brake does not drag/rub and that the disc brake is not bent or warped.

It is difficult to check the condition of drum brakes without taking them apart. With a drum brake it is a case of riding the bike and listening for any metallic noises indicating wear in the brake shoe.

It is advisable to then check for any wear in the wheel bearings. Simply feel for notchiness as it rotates and "wiggle" it from side to side, looking for lateral play in the bearings. Wheel bearings are not difficult or expensive to replace but they are another bargaining tool to help drive down the price of the bike for you.