Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Witty, Tongue-in-Cheek, Satirical, Ironic
It would be impractical to identify the tone of each of the 100 stories of The Decameron, and it won't be on the exam, so we'll start with the frame narrative and the author.
We're most aware of the author in this work in the Prologue, the Introductions to the First and Fourth Days, and in the Author's Epilogue. (Note: it's okay to speak of the author and the narrator interchangeably in this work. Just pick one and stick to it.) At these moments, he appears highly in control of his elegant prose and is obviously rhetorically polished—but then the content reveals his attitude toward his critics. He lambasts his critics and uses humor (some of it quite low) to defend his choices. Boccaccio's very serious when he describes plague-ridden Florence, but in those other moments, it's clear that he's having a lot of fun at others' expense.
Although the stories each have their own emotional tone, it's safe to say that many of them are also satirical and humorous. Some are laugh-out-loud funny and ironic. Much of the humor comes from the contrast between the high-mindedness of the characters in the stories and the way they succumb to their basest instincts. Supposedly devout clergy have ravenous sexual appetites. Ditto for the respectably married ladies. Everyone's succumbing to lust, greed, and deception while living in a society that preaches nobility and faithfulness. And of course, the contrast between the noble and refined members of the brigata vs. the shockingly bawdy stories they tell…well, let's say that it's hard to imagine they really slept as peacefully as they claim. On the other hand, maybe the extensive use of sexual euphemism protects the ladies' delicate sensibilities.
Some of the stories border on slapstick humor—people fall into wells and piles of excrement, get smeared with honey and feathers, get locked outdoors in the freezing cold, or are the butt of jokes and elaborate pranks. The Calandrino tales are classic examples of folktales about the town fools that you find in every culture.
There are dark and serious stories, too. Plenty of the characters in the stories are victims of lost love and tragic deaths. Some stories are just plain painful to read—the last tale, the one about patient Griselda, for example, is almost unbearable in its description of her husband's psychological torture. Death and disorder aren't far from the minds of the brigata no matter how much they try to escape it.