Its a difficult one, if its a car i dont really care about i give them the keys as long as they dont look dodgy and then say help yourself and i just stand there however i made the mistake before buying a car which i hadnt driven only taken for test drive which was fine until i drove the car home and found the clutch slipping, personally even without insurance id let anyone take my car around the block with me in the passenger seat as chances are they wont be an idiot if your in the passenger seat and if they start doing things you dont like tell them if they want to do it then they can buy the car first