Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Despite the sympathy Rhys expressed for Antoinette's situation in her letters, in the actual novel the prevailing attitude is critical. It's not that the novel disapproves of anyone necessarily, but it's trying to evaluate everything with a critical eye. There are no heroines or villains in the novel. The novel seeks to create a balanced view of the forces that push and pull each and every single character – it seeks to understand, not to praise or condemn. Rochester may be a loser for marrying a girl for her money, then locking her up in his attic, but the novel tries to give Rochester the benefit of the doubt by letting him tell the story from his perspective. Instead of an evil guy, we get someone who is vulnerable, naïve, sometimes well-intentioned, and not particularly brilliant, trying to make the best of an overwhelming situation. Antoinette may be the heroine and the inspiration for the story, but the novel also shows how her own limited way of looking at the world contributes to her unhappiness.