You can often infer the theme of a piece of literature by ...

Consider the author's opinions. The theme is the universal message or moral of a literary work, therefore you can use textual evidence to determine what the author intended to tell the audience. For example, the theme of "The Boy Who Cried Wolf" is that lying can give one a reputation never to be trusted. Readers know this because the boy continuously lies, and the end result is that no one believes him the one time he was telling the truth. From that evidence, we can infer that the author believes people should not lie.