To determine the reason an author wrote a text, the reader must study information about the author’s life. the descriptive details in the text. other sources about the same topic. the table of contents of the text.

To determine the reason an author wrote a text, the reader must study the descriptive details in the text.

Types of things a reader could look at in a text to help find the writer's aim or purpose are:

Conversations between characters...what do they say? 
Narrative that explains a point of view.
Figurative language that reveals meaning and insight about a topic.
Setting...when and where does a story take place? Does this reveal other mindsets?
Ideas that are presented in narrative or by the characters.

There are many ways to figure out what an author is trying to share with his or her audience. Paying attention to the details will help a reader infer meanings and come up with supporting evidence to show why and author wrote a particular piece.