Before the keys change hands, ask the seller to leave the engine cold. Start it yourself, try both windows and the air conditioning, and make sure you can get the seat and mirrors where you want them. At parking speed, pay attention to clutch take-up or the automatic’s response when you ask for a small amount of movement.
Find some broken tarmac before the nice bit of the test route. Listen for rattles and see how the car settles after a bump. Try reversing it somewhere you can actually judge the view. If the dealer says a noise or a warning light will be sorted before collection, put that on the order.
Write down what you found before you look at the next advert. It’s much easier to compare two cars when you can remember the details you’ll have to live with.