Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Moralistic, Introspective, Tongue-in-Cheek
Twain's has a point to make and he's going to get it across, with the story's plot line as well as through Huck's explanation of his inner thoughts. Here's a good tone example from Chapter 31:
I felt good and all washed clean of sin for the first time I had ever felt so in my life, and I knowed I could pray now. But I didn't do it straight off, but laid the paper down and set there thinking—thinking how good it was all the happened so, and how near I come to being lost and going to hell. And went on thinking. And got to thinking over our trip down the river; and I see Jim before me, all the time, in the day, and in the night-time, sometimes moonlight, sometimes storms, and we a floating along, talking, and singing, and laughing. But somehow I couldn't seem to strike no places to harden me up against him, but only the other kind. (31)
But besides Huck's many moral quandaries and important, life-changing decisions, there's also a ton of humor in the novel. Twain was a master of irony, wit, sarcasm, and satire, and the Adventures of Huckleberry Finn is riddled with all of them. Twain drops all kinds of irony into the plot, but one of his favorite tricks is showing that Huck isn't nearly as smooth as he thinks he is—like when Mrs. Judith Loftus, after hearing Huck spin a whole roomful of lies, looks him "straight in the face, and very pleasant, and says, 'Come, now, what's your real name'" (11).
But Huck isn't the only one whose self-image is a little inflated. The duke and the king say they are a real English Duke and the rightful King of France (the "lost" son of the assassinated Louis XVI)—but know through Huck's description that neither of them is even remotely believable as a European aristocrat, like when the young man asks the old one "What're you alassin' about?" (19).
And if that doesn't make you LOL, then you should probably go back to watching cat videos.