Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Gloomy; Reflective
Sister Carrie is the perfect book to curl up with on a rainy, dreary day. That probably has something to do with its gloomy tone:
Once the bright days of summer pass by, a city takes on that somber garb of grey, wrapt in which it goes about its labours during the long winter. Its endless buildings look grey, its sky and its streets assume a somber hue; the scattered, leafless trees and wind-blown dust and paper but add to the general solemnity of colour. (10.19)
Indeed, our narrator never fails to point out the darker aspects of life, which reinforces and adds another layer of bleakness to the already dark situations depicted in the novel. As Carrie grows more successful in the world's eyes, it's as if we're not supposed to be completely comfortable with that transition and the somber tone serves as a constant reminder of that.
The narrator frequently treats us to little philosophy lectures, too. Lucky us, right? Whether you read that last sentence with a sarcastic inflection or not, these digressions create a reflective tone, as in the following:
Thus in life there is ever the intellectual and the emotional nature—the mind that reasons, and the mind that feels. Of one come the men of action—generals and statesmen; of the other, the poets and dreamers—artists all. (47.116)
This tone really gets us in the mood to think about the significance of the more mundane character actions, like when Hurstwood reads the newspaper for the gazillionth time.