One can tell it's fiction because there is no evidence to suggest that Holden Caulfield was a real person. Also, the plot is full of the kind of events and emotional turmoil that are typical of fictional coming - of - age stories. The settings and situations seem crafted to convey certain ideas about youth and society, which is a characteristic of fiction writing. Non - fiction usually has a basis in real facts, historical events or real people's lives, which is not the case here.
It's mainly the nature of the story. Since it's about a made - up character's journey and experiences, it's fiction.