Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Why not start with a quote from our spunky narrator?
I didn't tell him that the diagnosis came three months after I got my first period. Like: Congratulations! You're a woman. Now die. (2.13)
And hey, how about another?
That particularly galled me, because it implied the immortality of those left behind: You will live forever in my memory, because I will live forever! I AM YOUR GOD NOW, DEAD BOY! I OWN YOU! Thinking you won't die is yet another side effect of dying. (21.8)
Oh, and why not one more—just for good measure:
I went to Support Group for the same reason that I'd once allowed nurses with a mere eighteen months of graduate education to poison me with exotically named chemicals: I wanted to make my parents happy. (1.28)
Notice a trend? Our girl is remarkably frank. She tells it like it is, even when other people don't like it (read: her mom). She knows she's headed for an early death and feels like there's not much use in pretending otherwise.
As frank as she is, though, she's also cautious. Even when she's dealing with something as harmless as answering Augustus's parents' questions about Support Group, she weighs her options carefully:
I paused for a second, trying to figure out if my response should be calibrated to please Augustus or his parents. (2.32)
And it's not only for Augustus's parents. She's also especially cautious when she's talking to or thinking about Augustus. She doesn't want to admit to her feelings for Augustus outright and doesn't like to consider a wonderful future together. She can't afford to get her hopes up when the stakes are so high, and her word choice makes that abundantly clear.