Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Dark, Moralizing
Although Richardson occasionally throws us a funny bone (what's up, Dorcas-the-toad?) the main tone of the book is dark and moralizing. It's the kind of story where you're always waiting for the other shoe to drop. Remember, Richardson got reamed for easing off on the moral stuff in Pamela, so Richardson isn't about to give us a happy ending here.
But even so, the dark moments at the end of Clarissa are really dark. Think of Clarissa's extended death scene: "We thought she was gone; and each gave way to a violent burst of grief" (481.18). And that comes after a whole two or so volumes of dying.
And then there's the capital-M Moral. Although it's more than a little cringeworthy for a twenty-first-century reader, we have to remember that morals were Richardson's bread and butter. Let's just hear from Lovelace at the point when Clarissa is definitely on her deathbed: "Tell her, oh tell her, Belford, that…I can and do, repent—and long have repented—Tell her of my frequent deep remorses" (470.6). In other words: repent, ye sinners; and ladies—don't let yourself be raped, but if you do, make sure manage to die without actively killing yourself.
The moral might fall short today, but it went the distance back in the day.