Usually it looks straight forward as to who is the protagonist of a story, but that’s really something that can be a little slippery.
Many people, writers included, easily mistake a viewpoint character for the main character or protagonist. George R.R. Martin has said in interviews that he was inspired by the movie Alien, which kept the protagonist hidden in plain view, for his epic series A Song of Fire and Ice. Who’s the main character in A Game of Thrones? We don’t know yet and that is because we have such a large number of point of view characters.
But even when there is a very limited number point of view characters identifying the protagonist may still be difficult.
In the film Ferris Buller’s Day Off there is no doubt that the viewpoint character is Ferris, aside from a few scenes here and there everything we see and hear is from Ferris’ viewpoint, but he’s not the main character. He’s just the person telling us the story.
To my way of thinking the main character is the person, or persons as it can be more than one who, over the course of the story, goes through the greatest change. I think ideally the character should take an action that would have simply not been possible for them before the events of the story, In Ferris Buller’s Day Off I think it is clear that Cameron is the main character. His actions over the car and what that represents in his relationship with his father are a dramatic change and growing for his character while Ferris leaves the story exactly the same as he entered it.
When you are looking at your story think about what the character can and cannot do. I do not mean physical powers or ability either, I mean what actions do their nature inhibit and look there for the real center of the story and for your protagonist.
A word of warning however. Do not be too slavish in the application. Rules in art are rarely unbroken. For example in most detective fiction the continuing characters rarely change. Holmes and Watson remain Holmes and Watson, at least for the most part in the original source material, and are not subject to a great deal of character change.
