One way is to look at the author's background. If the author is a historian or journalist known for dealing with real - life events, it may be more likely to be fact. But if it's an author of fantasy novels, it's probably fiction.
Check the sources. If the book references historical archives, research papers, or real - life interviews, it's more likely to be fact. If there are no such references and it's filled with made - up places and characters, it's fiction. Also, the genre classification can be a hint. If it's in the non - fiction section of a library or bookstore, that's a good sign it's fact - based. However, sometimes books are misclassified. So, reading reviews from reliable sources can also help.