This is under the presumption that the fire has proper ventilation by other means and did not self extinguish because of lack of oxygen. Out of thousands of door glasses that I have ever replaced, I have never seen where the door was free of large quantity of broken glass inside the door itself. Obviously if totally burned, these cannot be inspected. When a vehicle is on fire, electrical systems short. The horn comes on, the lights come on and other accessories work.
Power windows can go up and down during a fire.
Being that a vehicle is not secure and there are many different methods of entry, unless the obvious is seen depicting that the vehicle was forced entered before the fire, there are very few situations where one can possibly tell how a totally burned vehicle was last entered. It is relevant to look for every possible means of forced entry however, if none of the obvious signs are not found, forced entry cannot be ruled out in a totally burned vehicle.