Having a mechanic with you for the test drive is half the battle won!
It is not often that you can do this, but if you make a decision to buy a used car, and you have a reliable mechanic at your service, invite them along to test drive the car with you. This will help you understand the potential problems that you may have with the particular car almost instantly. If you do not have a mechanic available, do not ask the used car dealer to allow you to have a chat with their mechanic as this is more likely than not going to help your cause.
Your mechanic will be able to highlight minor imperfections on the vehicle’s body and will usually pop open the hood in order to have a look at the engine. This may make the salesperson uncomfortable but it is your right to do this before making a decision to purchase the car. The mechanic will also test the brakes and other car parts as far as possible and will check that the spare tire and other accessories that normally come with the car, for example, the car kit are there. If after doing all of this, your mechanic rolls his eyes at you, wish the salesperson a nice day and walk away!