If you are an expecting parent, buy your car seat before shopping for a car. Not all back seats will properly accommodate a car seat, so make sure you bring your car seat to the dealership and try installing it. A correct and tightly installed car seat will move no more than an inch in any direction.
Stow the stroller to make sure the car has enough storage space to accommodate your stroller or carriage plus other items (shopping, groceries, etc). Lift the stroller in and out a few times as if you will be doing it often.
Check the rear windows. Some sedans and coupes have deeply-sloped rear windows that allow sunlight to stream down on a rear-facing child seat and possibly in your child’s face.
Take a ride in the back. From time to time, you (or your spouse/partner/doting grandparent/etc.) may want to ride next to baby, so part of your test drive should include a ride in the back seat next to the strapped-in child seat.