Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Hopeful Despair
There's tons o' death and decay in The Waves. We look on as the narrators age, endure the death of two friends, and often fixate on all the stuff that is rotting and/or dying (this book makes windfall apples sounds really nasty).
For example, when Susan and Bernard are running around Elvedon and they come across the lady writing in a window, Susan turns a benign or even peaceful image into a sinister one, remarking, "I see the lady writing. I see the gardeners sweeping [...] If we died here, nobody would bury us" (1b.49). That dark little gem, which comes pretty much out of the blue, is just one example of how Woolf peppers the novel with references to death, often to jarring effect.
Yet despite these frequent references to the darker side of life, the novel ends on a hopeful note with Bernard's promise to rail against the forces of death and decline. Hecky yeah, Bernard! Do not go gentle into that good night! Prior to his decision to fight with all his might, he's actually feeling pretty down in the dumps and fretting that his lifelong obsession with phrase collecting has been pretty much meaningless. He says,
My book, stuffed with phrases, has dropped to the floor. […] What is the phrase for the moon? And the phrase for love? By what name are we to call death? I do not know. I need a little language such as lovers use, words of one syllable such as children speak when they come into the room and find their mother sewing and pick up some scrap of bright wool, a feather, or a shred of chintz. I need a howl; a cry. When the storm crosses the marsh and sweeps over me where I lie in the ditch unregarded. I need no words. Nothing neat. Nothing that comes down with all its feet on the floor. None of those resonances and lovely echoes that break and chime from nerve to nerve in our breasts, making wild music, false phrases. I have done with phrases. (9b.79)
However, before you even have time to get alarmed, Bernard seems to collect himself, possessed by a renewed zest for life (and, presumably, language, since he appears to be feeling talky once again). He says,
I am aware once more of a new desire, something rising beneath me like the proud horse whose rider first spurs and then pulls him back. What enemy do we now perceive advancing against us, you whom I ride now, as we stand pawing this stretch of pavement? It is death. Death is the enemy. It is death against whom I ride with my spear couched and my hair flying back like a young man's, like Percival's, when he galloped in India. I strike spurs into my horse. Against you I will fling myself, unvanquished and unyielding, O Death! (9b.84)
Presumably, this renewed desire means he won't be giving up on language and communication—the twin concerns that had been making him gloomy just a moment ago—any time soon. That's a pretty uplifting ending, if we do say so ourselves.
See: there's really no need to be afraid of Virginia Woolf.