Tone is generally determined by several factors. In Langston Hughes's poem "Harlem," tone can best be determined by his use of imagery. This work has a simple structure: it is a series of images which draw a picture of the answer to a question he asks but never answers: "What happens to a dream deferred?" The images that follow are possible answers to that question and full of sensory details which lead to a feeling of hopelessness.
Does it dry up
like a raisin in the sun?
Here we have the visual and tactile image of something once full and ripe which is now shriveled and dried up--like a dream deferred.
Or fester like a sore—
And then run?
This image is one of a partially crusted but mostly open sore which has not healed; instead, it oozes its ugly fluids and never really heals--like a dream deferred.
Does it stink like rotten meat?
The image here is one of a life-giving substance which is now inedible and has the look and smell of decay--like a dream deferred.
Or crust and sugar over—
like a syrupy sweet?
Here I think of something like maple syrup left to dry up on a plate--hard and crusty and no longer smooth and usable--like a dream deferred.
Maybe it just sags
like a heavy load.
This image is one of pure weight, a burden which will not be lifted. Visually, the word "sag" depicts an utter hopelessness--like a dream deferred.
Or does it explode?
Finally, all the senses are invoked in this image--sound and sight and touch and smell and even taste, as the smoke and dust of an explosion fills the mouth. There is nothing left; all is lost--like a dream deferred.
This structure of rhetorical questions replete with sensory images and details is designed to create a growing sense of hopelessness, which it seems to me is both the tone and the theme of this poem.