Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
This is hardly surprising, since the novel is composed of a series of personal letters and journal entries. Supposedly, the writers of the letters and journals didn't intend them for anyone else's eyes as they were writing them. Some of the letters and journal entries include some pretty personal stuff. But that personal tone adds to the illusion of realism in the novel: the documents we're reading seem more legit because we can imagine real people (as opposed to fictional characters) writing them.