There's more to it than this but very simplified:
A fearful dog is reacting to a non threatening situation or thing.
A protective dog reacts to an actual threat. (this can include a perceived threat by you if you are actually scared).
Is the dog reacting to a random person walking up to the car or near you on a walk posing no threat and it causes a reaction? If that's the case, it's probably fear.
Is someone threatening or yelling at you and your dog intervened? If that's the case, it's probably protection.
To be honest, if you're not sure, I'll bet 99% of the time it's fear.