Stretch a bungee/rope from concrete to eaves on each side of the door, slightly inset from the opening (and a bit further out) such that the car will hit the line bungee before it hits the wall. Attach a ball or something cute to the middle of the line so that you can see it jiggle if you hit it.
Once you have the full nose of the car in its not useful (because you wont be able to see it), but if you manage to smash the side of your car you probably deserve it. Use additional guides inside the garage if you need to.