How is the main character observant

Well, It depends.

Explanation:

If the main character is the narrator of the story, (Meaning it is a first-person point of view story) than the fact that he is telling you about whatever makes him observant. In other words, he is observing the action and describing it to you piece by piece. If the story is in third-person, you would have to explain the role of the character and his/her personality traits to answer the question. (The same applies for a second-person story.)

what passage are you referring to