Foreshadowing is a device used in literature in which the writer gives the reader a hint about something that will happen later in the story. There are many different forms of foreshadowing—it can be obvious or subtle, and can occur throughout a story, although it can usually be found in the beginning. The purpose is to grab the reader's attention, engage him in the story and get him to make predictions about what's going to happen. Foreshadowing can be in the form of a scene or action, a sign, symbol or omen, or something a character says. For example, a character could hear the sound of thunder before something bad happens.
As a reader, a good way to identify an example of foreshadowing is to think about it from the author's point of view. Ask yourself: why would the author include this object or have this happen? Stop and ask yourself questions about the purpose of the characters' actions or words and try to notice when something really stands out to you or is emphasized a lot in the text. An example of foreshadowing in "Before the End of Summer" occurs in the beginning of the story when 10-year-old Bennie overhears a conversation between his grandmother and her doctors in which they tell her that she is going to die by the end of the summer. She herself also makes some comments that give both Bennie and the reader a clue that she knows she doesn't have much time left.