Now that you are comfortable and have set up everything to your liking, mirrors adjusted, seat and steering wheel set, turn down the radio. Yes, I know we want to hear how that sound system cranks but you also don't want to miss the road or excessive wind noise. Besides, it's a new car and will handle different, you don't want to be distracted.
Now make sure you take the car out in conditions you drive in daily. Are you in stop and go traffic, do you get on the interstate to commute? The dealer may have a small pre-determined route for a test drive, but if it doesn't cover what you need then feel free to ask to drive your own route. If you travel the interstate then make sure you get the car up to speed. How does it merge in traffic? Not something you want to find out after a Semi is bearing down on you!
Next pay attention to how it handles. How does the steering wheel handle. It it tight and stiff or loose with play? When taking a corner does the car stick to the road or can you feel the body roll? Next how does it handle bumps or rough roads? When hitting a bump or pothole listen to any strange squeaks or noise from the chassis. Is the ride smooth or will it make you loose your stomach. I find nothing worse, than finding out on a long road trip that my car leaves my body feeling beat and bruised. If this is important to you, then make sure we find out before we buy. Also, with all the new technology many cars come with
Last but not least! I said "drive it like you own it" but lets not give the salesperson a heart attack. Test the accelerator, hit the brakes to see how it stops, (you may want to let the salesperson know in advance) but keep in mind this make not be the right car for you. Treat it with respect for the person that may find it's the right car.