How would a driver know that a brake-light bulb is not working?

They would notice the brake pedal become firmer under pressure.
They would notice the brake pedal getting softer under pressure.
By standing at the rear of the vehicle and checking as another person presses the brake pedal.
By standing at the rear of the vehicle and getting another person to turn on side lights.

Explantion: In order for a driver to know that the rear brake lights are working correctly, they should stand at the rear of the vehicle and check that the brake lights come on while another person presses the brakes.

Theory Tester

How would a driver know that a brake-light bulb is not working?

Legal Matters/Rules of the Road