Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Brooding, Sarcastic
Holy snark, Batman. The tone of This Side of Paradise often sounds like a parody of a young man who thinks he is super deep and has a lot to say. It's often dark and brooding, and super-sarcastic in its most negative moments.
Take a gander at this barbed stormcloud of a line:
Amory's two years at St. Regis', though in turn painful and triumphant, had as little real significance in his own life as the American "prep" school, crushed as it is under the heel of the universities, has to American life in general. (1.1.226)
Or in other words, Fitzgerald is saying both "So Amory went to a snobby prep school. Woopty-doo!" (that's your sarcasm, there) and "The prep school experience is meaningless. What is meaningful in this life, after all?"
This tone continues through the rest of the book, as Fitzgerald piles up more and more difficult life questions… without answering any of them.