Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
As a narrator, Chaucer shifts between appearing very naïve (i.e., inexperienced and way too ready to believe whatever anyone tells him) and approaching his subjects with heavy irony, or knowledge about the difference between the way the pilgrims want to appear and the way they actually are. We see this all-believing, or credulous, tone most often when Chaucer praises pilgrims. For example, in his portrait of the Monk, we learn that the Monk believes that he should let "old things" – like his vows of poverty and chastity – pass. Chaucer tells us, "I seyde his opinioun was good / What should he studie and make hymselven woode?" (General Prologue 183 – 184). Chaucer's easy acceptance of the Monk's excuses here make him appear a little naïve as a narrator, and as a character.
On the other hand, in the Prioress's portrait, Chaucer slyly exposes the difference between how the Prioress wants to appear (as a high-class lady) and what she actually is (a religious figure trying to appear to be a high-class lady). We should mention here that literary types don't always agree about when Chaucer's being credulous and when he's being ironic. Take that example of credulousness in the Monk's portrait. Well, some people think that there, Chaucer's repetition of the Monk's opinion actually makes it appear ridiculous. Yeah, the tone is pretty complicated. That's what makes it so interesting.