How can you determine when a protagonist is presented directly or indirectly in a given story?

This is an issue of knowing the types of characterization.  First you have direct characterization, which is done through the techniques of giving vivid descriptions and using direct narration (storytelling).  Then, you have indirect characterization, which includes the following techniques: using dialogue with other characters to give information to readers about a character, using that character's actions to help readers understand him/her better, and finally, the use of other characters' comments about that character in the story in order to make clear the attributes of the character in question.

So, it's really an issue of figuring out how the author is presenting the character.  Which of the techniques of characterization as listed above does the author use to tell you about the protagonist?  Once you determine that, figure out whether it falls under the umbrella of direct or that of indirect.  It's as simple as that!  Although, you may want to have some quotes available to back up your opinion.  Good luck!