The death of Homer Barron is foreshadowed in the interlude describing the terrible smell which came over Emily's property. The phrasing allows a connection to be made by the reader without any implication that the same connection might have been made by the narrator, or the townspeople, at the time—indeed, it seems from the tone of the narration that the fact that Emily's "sweetheart...had deserted her" without anyone knowing his whereabouts was seen as almost incidental. This is simply a means of placing the incident of the smell in context in the story of Emily's life.
The "ladies" decide that the smell must have come from Emily's kitchen now being the purview of one lone black servant. Eventually, the smell becomes extremely bad, and they begin to assume it might have been some animal "dead in her yard," but the social structures of the town are such that nobody wants to actually speak to Emily about it. She is not the same class as the others, and she holds a particular position in the town. As such, the people decide to take on the smell themselves. They sprinkle lime in the grounds of Emily's house, and shortly thereafter, the smell goes away.
At this point, the narrative moves on from the smell, onto the idea that Emily may be going crazy like her relatives, but a seed has been planted in the reader's mind. When the story goes on to explain who Homer Barron really was, up to the point of Emily going to the shop to buy some poison, the reader puts two and two together. The townspeople think that Emily is going to kill herself, but when Barron is suddenly gone, the reader—at least, in my case—becomes sure of what has happened, earlier suspicions confirmed.
Did you guess the ending at this point? Earlier? For me, it simply heightened my curiosity to know what was really going to happen, what Emily had done with the body of Homer Barron, and why. It's up to you to say how these clues made you feel about the story as it developed.