Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Lighthearted
Lots of nightmarish events happen in The Three Musketeers. Several people are killed, including our hero’s true love. The people of La Rochelle are starving to death. The Queen is stuck in a loveless marriage and is being hounded by a guy she rejected. On the face of it, the subject matter of The Three Musketeers isn’t exactly Saturday morning cartoon material.
Yet Dumas brings an incredibly lighthearted touch to the story, milking every comedic moment and rapidly glossing over the loss of human life. Everything is a fun adventure to our heroes, who quite willingly risk their lives over issues as trivial as a funny-looking horse:
This time there could be no doubt; D’Artagnan was really insulted. […] Unfortunately, as he advanced, his anger increased at every step; and instead of the proper and lofty speech he had prepared as a prelude to his challenge, he found nothing at the tip of his tongue but a gross personality, which he accompanied with a furious gesture.
"I say, sir, you sir, who are hiding yourself behind that shutter--yes, you, sir, tell me what you are laughing at, and we will laugh together!"
The gentleman raised his eyes slowly from the nag to his cavalier, as if he required some time to ascertain whether it could be to him that such strange reproaches were addressed; then, when he could not possibly entertain any doubt of the matter, his eyebrows slightly bent, and with an accent of irony and insolence impossible to be described, he replied to D’Artagnan, "I was not speaking to you, sir."
"But I am speaking to you!" replied the young man, additionally exasperated with this mixture of insolence and good manners, of politeness and scorn. (1.14 – 1.17)
Yes. This scene is basically a precursor to Taxi Driver's "Are you talkin' to me?" scene. Except that it's motivated by some dude laughing at a hideous yellow horse... instead of one cabbie's dangerous mixture of sociopathy and PTSD.