Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
One of the things that has made The Lion, the Witch, and the Wardrobe such a successful book is C.S. Lewis's ability to balance his didactic message about the Christian faith with a lighthearted tone. Although the book often has clear moral messages to convey, and Lewis isn't about to let anybody miss them, it is often amusing and sometimes silly. At times, the description of pleasant things – such as tasty meals or beautiful natural scenes – takes on a life of its own, going on and on, making the reader positively long to have tea with Lucy and Mr. Tumnus, or walk through the summery Narnian countryside. With these beautiful evocations of the senses, the reader probably is more willing to think about the morals and ethics that Lewis is pushing.