Scratches, dents, condition of glass, look underneath for any leaks scuff marks on wheels. Make sure all 4 tires are the same.
Open the hood, look for any modifications, loose wires, how clean the motor is or did they just put alot of Armor all on everything.
Make sure all lights work, check signals, backup and headlights.
Make sure all the seats work, no tears in the carpet.
Start the car, how does it idle, run, look at cluster for any abnormal warning lights, Check engine lights.
Look at exhaust system, been altered ? sealed good ?
Drive it, does it pull to one side, is it noisy, turning smooth ?
List goes on forever.