Foreshadowing is when one event or reference in a narrative hints at what is to come later on in the story. "A Good Man is Hard to Find" has a few examples of this literary technique.
Firstly, the grandmother mentions the Misfit before he makes his first appearance. She reads about the Misfit's prison break in a news article and shares the information with the rest of the family during their road trip. This article provides both exposition regarding the Misfit's background (he is a dangerous criminal who recently escaped from a Florida prison and is currently at large) and foreshadows the family's fatal run-in with him on the road.
Secondly, the deaths of the family members are foreshadowed before they meet the criminals. During an extended description of the grandmother's wardrobe, the narrator ends the section with this ominous statement:
In case of an accident, anyone seeing her dead on the highway would know at once that she was a lady.
This not only highlights the grandmother's superficial nature, it also foreshadows her ultimate fate. She literally will be dead on the side of the road by the conclusion of the story, though by then, she will be utterly transformed due to her encounter with the Misfit.
During a description of the family's travels, they also happen to pass a few graves in the middle of a cotton field. There are specifically "five or six" of them, which is roughly equal to the amount of people in the family. This too foreshadows the violent fate of these characters.