Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
The narrator of "The Reeve's Tale" doesn't spend a lot of time commenting upon the things he's telling us, despite the fact that what we learn about the characters is sometimes shocking. For example, we learn that Symkyn regularly cheats his customers with "A theef he was for sothe of corn and meele / And that a sly, and usuant for to stele" (85-86). That's it, and the narrator makes no comment upon this piece of information, quickly moving on to discuss Symkyn's name and family connections. So, where we might expect outrage or moralizing, we get "just the facts, ma'am," plain and simple, a manner of delivery that's indicative of this narrator's straightforward style.
Sometimes the narrator does attempt to draw lessons from what he's just told us, like when he tells us that "jalous folk been perilous everemo" to explain Symkyn's aggressive possessiveness of his wife. Even this, though, is presented as a matter of fact, and not something that needs to be argued. This straightforward tone comes in particularly handy at the end of the tale, when it enables the narrator to interpret the rape of Symkyn's wife and daughter, and Symkyn's beating, as simple "payback" for the corn he stole. We're so accustomed by this point to accepting the narrator's words as plain statements of fact that we might not stop to question the validity of this interpretation, which is probably what the narrator's counting on.