It can be really, really difficult to tell the difference. There are some signs that are a little more obvious, like a tucked tail, cowering, or trying to hide, but not all dogs are that obvious. Another good indicator is how the dog reacts leading up and following the encounter.

My sisters dog is obvious fear based. If someone approaches him, he tries to leave. Even as he barks, he is backing away. He will stop barking and visibly relax as the stranger leaves. My dog is excitable. He sees people, and his barking is loud, high pitched, and increasingly frantic. When people walk away from him, it's worse. When he approaches people, or they approach him, and it's apparent that they'll meet each other, his barking stops, tail wagging, trying to sit but too wiggly, excited snorting, that sort of thing. But, with some dogs, it's way, way harder to tell. And over excitement can flip easily into other extreme emotions.

If you aren't certain, I'd recommend working a good trainer or behaviorist who can help you figure it out. They're both difficult behavior problems to deal with, so a trainer is a good idea anyway. Plus, issues can be really complex. For example, dogs can be fearful and aggressive only while being walked on leash by their owner.