Determine the nature of the content. If the book you are looking at is a biography, autobiography, historical text or deals with material that lists facts and figures, it is likely that you are reading a nonfiction book. If the book deals with mythical creatures, or uses facts but creates a dramatic storyline rather than researched facts, the book is probably fiction.
Look on the side of the book. Most books will list whether they are fiction or nonfiction accounts.
Evaluate whether the book is based on a person's life story that only changes the names of the characters, but keeps everything identical, or if the story is only loosely based on the writer's life. If the writer only changed the names, but stayed true to the facts, it may classify as nonfiction, provided this information is disclosed in the text.