Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Hyperbolic
The narrator of "The Man of Law's Tale" really loves to spin a yarn. And to do so, he relies on his fair share of exaggeration. Accordingly, Custance is not just beautiful and virtuous; she's the most beautiful woman who has ever been born "syn the world bigan" (159). The sorrow a character feels is not just sad, but so sorrowful that "ther is no tonge that it telle may" (899). This exaggeration shows us how involved the narrator is in the tale. Dude's invested.
Emphatic
The Man of Law throws his support behind Custance wholeheartedly, emphatically criticizing anybody and everybody who does her wrong. He rebukes the Emperor of Rome for failing to consult the horoscopes that might have predicted her suffering. He calls the Sultan's mother the "roote of iniquitee" (358).
This hyperbole and emphatic commentary exist for a reason, of course: the narrator wants to get the audience interested in his tale by appealing to familiar and passionate emotions like pity, anger, and sadness. If he can get them to feel strong emotions about his tale, he can get them to care about his characters' fates, guaranteeing that his audience will want him to keep talking.