Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
The comic exchanges between Charles and his misanthropic father are some of the most famous in the novel, or even in Waugh’s collective work. This type of sardonic humor pervades Brideshead Revisited. The novel ruthlessly satirizes the extravagance of the British aristocracy. Just look at Julia’s diamond-studded tortoise and the reactions it provokes: Lady Marchmain wonders if it eats the same things as an ordinary tortoise, and Samgrass wants to know if they can stick another tortoise in the shell when it dies. You’ve also got "life-size effigy of a swan, moulded in ice and filled with caviar," a "chilly piece of magnificence," in Charles’s words, "dripping at the beak." (Hilarious.) Waugh also mocks the careless attitudes of the spoiled rich, mostly through Boy Mulcaster, who rings a fire alarm one night in order to "cheer things up" at a boring nightclub.
But this wry wit it in constant balance with the novel’s melancholy nostalgia. This sadness really hits home in passages like this one:
How ungenerously in later life we disclaim the virtuous moods of our youth, living in retrospect long, summer days of unreflecting dissipation, Dresden figures of pastoral gaiety! Our wisdom, we prefer to think, is all of our own gathering, while, if the truth be told, it is, most of it, the last coin of a legacy that dwindles with time. (1.3.6)
Remember that the older, narrator Charles is telling the story, which means the attitude with which he reflects on his past experiences will largely define the novel’s tone. Because in the 1940s he is "homeless, childless, middle-aged, loveless," it is with a sense of longing that he looks back the "Arcadian days" of his youth. The novel may end on an "unusually cheerful" note (see "What’s Up With the Ending?"), but narrator Charles is melancholy for the all of his remembrance.