If you commute, drive the car in both stop-and-go traffic and at highway speeds. If you trek to the mountains, find some steep hills to climb. Drive over bumps, take tight corners and test the brakes in a safe location. Get in and out of the car several times and be sure to sit in the backseat, especially if you plan on carrying passengers. Check out the cargo space. If you plan on using children's car seats, bring those along to test for fit and ease of installation.
While you are evaluating the car, don't be distracted. Take your time looking everything over. A good salesperson will respect your need to experience the car and will let you focus on the driving experience. Turn off music so you can listen to the sound of the engine. You can evaluate the sound system when you return to the dealership. If the conversation does turn to questions about whether you'd like to talk about purchasing or leasing, you can say that you're still in the test-drive stage.