Since the story is so short, there isn't much time to slip a whole lot of foreshadowing in, but, Bradbury does manage it. Take a look at the first line. Right in the first line of the story, it mentions that there is a clock that speaks out the time to the house and family, and tells them that it is time to get up. Then, here is the foreshadowing; the clock speaks "as if it were afraid that nobody would." Right there that is foreshadowing. The clock, an inanimate object, is given a fear that no one would get up when it asks. That foreshadows that indeed, no one will respond to its call, which alludes to something terrible having happened.
Breakfast is then not eaten--this could reference an empty house. Then, when the clock chimes in again, stating that it is time to go to school, once again, here is the foreshadowing:
"But no doors slammed, no carpets took the soft tread of rubber heels."
Once again, there is a reference to an absolutely empty house, a house in which no one is going about their business, going to work and school. From here on out, signs of emptiness increase--no car leaves the garage, the uneaten food is thrown away. Then at ten o' clock, what has happened becomes much more clear--the story describes the house being the lone survivor in a ruined city. At this point, we begin to understand what might have happened, and then right after that, we get the description of the images on the side of the house, images of the family that used to live there, that now does not because they were decimated.
So, the foreshadowing comes very quickly in the story, and in small ways until the full devastation is revealed; I hope that helps! Good luck!