Once inside the car, take note of all the location of all the safety features like windshield wipers, headlights, horn and so on. It's amazing how many people climb into a strange car and take off without knowing where these switches are and so they start fumbling for them out on the road where such a distraction can be very dangerous.
While the car is stopped in a safe area, it's a good idea to get in and out of the car a few times and make sure that it is relatively easy for you to do so. Also don't forget to check how easy it is to enter and exit the car from the back seat. If you have a family make sure that they will fit comfortably in the back seat as well as the front.
An important consideration for tall drivers is to check the feel of the driver's seat, especially if it goes back far enough to comfortably accommodate your height.
It's often a good idea to take the car to a deserted parking lot and test the brakes and cornering more thoroughly and aggressively than you can on the road. Just do it in a safe manner that isn't reckless. And keep the radio off as you drive to be able to listen closely for noises that could signal a possible problem with that vehicle.
If you practice these suggestions when you test drive a new car it will help you get thoroughly familiar with the vehicle and how it fits you and your needs. If it does not measure up to expectations expand your original search instead. But only buy a vehicle that you feel totally comfortable about after the test drive.