Low-mileage cars are worth more than high-mileage ones, which is why some vendors reduce the displayed mileage on the cars they're selling – it'a a practice known as clocking. A car's mileage display is known as its odometer; many modern ones are digital so they're even easier to clock.
However, traditional analogue odometers have to be removed for the mileage to be wound back, so if the car has one of these, look for evidence that the dashboard has been tampered with. Damaged screw heads is one way of looking, or scratches in the paint around the screws.
Whatever type of odometer is fitted, check that the wear and tear on the car fits in with the stated mileage. If the pedal rubbers and steering wheel are worn smooth, the car isn't a low-mileage one. Ask for the car's service history and previous MoTs; they'll all have the mileage on, so make sure it goes up steadily and doesn't suddenly drop.
It has also been known for a car's mileage to be reduced for the selling process, but once you've snapped it up, the odometer then mysteriously reverts to its true reading. That's why you need to check the reading doesn't suddenly shoot up between buying the car and collecting it.