Ob give it a good look over etc body work look at the tyres check the brakes/ oil level also the colour of it if it really black shows not been serviced for a while/ run the engine up to warm check the coolant also blip the throttle and see if smoking from exhaust basically. Also when test drive go through all the gears including reverse go from lock to lock check that there no splits in the cv joints etc Seems a lot a mechanic by trade and I just go over the car with a fine tooth comb always managed to get money off as well