Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Dickens uses a lot of really sharp irony in Oliver Twist to satirize the various institutions (the parish workhouse system, the justice system, the poor laws, etc.) that he thought were inhumane and unjust. For example, at the start of Book II, Chapter Five, the narrator satirizes Mr. Bumble by sarcastically calling himself "a humble author" in comparison to "so mighty a personage as a beadle." In actuality, of course, a beadle wasn’t all that important of a person, so Dickens is clearly being ironic. The effect of the satire is to show how pompous and self-satisfied officials like Mr. Bumble are really just full of it.