Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Mocking, Passionate, Melodramatic
The tone of Les Liaisons Dangereuses varies with its different narrators.
Madame de Merteuil and Monsieur Valmont are often playful and mocking when writing to one another. "Truly Vicomte, you are intolerable," the Marquise writes to him. "You treat me as casually as though I were your mistress. I shall be angry, you know" (2.51.1).
Cécile and Danceny, who are genuinely in teenaged infatuation, write to each other with passion when things are swell and, in Danceny's case, high melodrama when everything isn't proceeding as he'd like. "Oh, Monsieur!" Danceny cries to Valmont in an overstated letter. "I am desperate. I have lost everything" (2.60.1). He's like that a lot. We're not sure what Cécile sees in him.
Madame de Merteuil also has a flair for melodrama, but hers is very deliberate, not authentic. Here's a little exchange with her latest lover:
I put my arms around him and sink to my knees. "Oh my dear," I say, […] I am sorry that I could for an instant have veiled my heart from your gaze. Forgive me, and let my love make amends for my sin." You can imagine the effect produced by this sentimental speech. (1.10.7)