Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Though "Zooey" is narrated by Buddy Glass and "Franny" is – as far as we know – narrated by some uninvolved third person, the style and authorial tone are both essentially continuous throughout. The continuity of the two short stories is part of the reason why they can function effectively as a novel. We can see rather clearly in "Franny" that the author shares his title character's disdain for Lane-like pretension. This is evident even in the first paragraph describing the college boys "standing around […] talking in voices that […] sounded collegiately dogmatic, as though each young man, in his strident, conversational turn, was clearing up, once and for all, some highly controversial issue, one that the outside, non-matriculating world had been bungling, provocatively or not, for centuries" (Franny.1.1).
In "Zooey," there is no such obvious whipping boy for authorial judgment; instead, the author puts forward his Glass family as the positive counterbalance to what was negatively portrayed in "Franny." According to "Zooey," most of the world is silly and materialistic; but the Glass family is a special exception, to be admired and revered.