Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
All of the narrators imagine themselves to be writing to some unknown future reader, because Franklin Blake's stated purpose in putting together the documents is to tell the story for future generations. As a result, the narrators find themselves addressing the reader directly, periodically asking "what would you have done" and appealing to the reader's sympathy. The result is that the tone of the novel is very familiar – the narrators (especially Gabriel Betteredge) tend to treat the reader like an old friend.