Hints for buying a car
There is a chance that the car you test drive will have underinflated tyres. Although this is bad for safety and fuel efficency, it will give you a smoother ride and that can be helpful when making a sale.
Although you could bring along a tyre pressure gague in your pocket, it can be easier just to look at the tyres and if they appear at all underinflated you can ask them to put some more air in them. Unless your salesperson is Mister Grumpypants they will probably bend over backwards to please you. Then, while they're contorting their spines by bending over backwards, grab their air pump and top up the tyre pressure.