Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Measured, Even
Even when things go badly for our protagonists—and we mean really, really badly—there's a kind of stoicism in Steinbeck's narration that propels the story forward. This is a drama-free zone, so much so that when Jim dies a horrific death, the narrator pretty much keeps it real. Mac and London simply carry on, without judgment from the author:
London saw them at last. He came close, and stopped; and the lantern made a circle of light. "Oh," he said. He lowered the lantern and peered down. "Shot-gun?" (269)
This emotional detachment certainly highlights the numbness that the men feel at the general dehumanization caused by a steady dose of oppression and violence. But it also makes us feel that what we're reading is somehow real or true, or at least truthy.