Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Updike is a serious writer, who's known for tackling all sorts of tricky topics like religion, alcoholism, and spousal abuse, just to name a few. His trick is to delve headfirst into an issue, but to do so in a humorous manner.
In "A&P" the serious issue is conformity. Although we would argue that a certain humor is maintained throughout this brief tale, it's tinged with darkness. And despite Sammy's victory, it ends on a note of dread and isolation. This lends to the story's realism – after all, Sammy has just experienced a kind of trauma. He's angry from seeing the girls humiliated and also frightened about what his act of daring (which nobody seems to be patting him on the back for) means for his future.