Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Playfully witty, at times somber
Forster has a gift for being playfully witty. His sense of humor is never super exaggerated: it's completely deadpan, which is what makes us laugh even harder.
Early in the novel, Mrs. Herriton says that when a man is neither "well bred, nor well connected, nor handsome, nor clever, nor rich" (1.31), even Lilia will lose interest. But then Philip comes back with the comment, "No, I believe she would take any one," which translates to "Lilia is desperate and will chase after anyone with a pulse."
The fact that Philip is perfectly serious is what has us completely in stitches. Forster is mean and deadpan: the Jim Gaffigan of Edwardian writers.
However, even though we spend a lot of time making fun of the flaws and silliness of the characters, we can't help feeling somewhat sympathetic to their problems—after all, no one is perfect and the somber truth of Forster's novel is that we all suffer from the same kinds of human weaknesses. Bummer.