First, we are told that the house at which Amir plans to meet the Taliban official is a "compound" on "Street 15, Sarak-e-Mehmana, Street of the Guests"--Baba's old house, the same one in which Amir had grown up. When the Talib official asks Amir,
"Whatever happened to old Babalu, anyway?"
the reader, like Amir, becomes aware that the official knows Amir's family from sometime in the past. Another hint is given when Amir sees the man's "bloodshot blue eyes": It is Assef. We discover that Assef still wants to settle the old score between them--and violence is imminent--when he tells Amir
"I heard your father died. Tsk-tsk. I always did want to take him on. Looks like I'll have to settle for his weakling of a son."
The author's mention of the "brass balls" foreshadows their later usage, and when Assef removes his "brass knuckles," the reader knows that Amir will be in a fight for his life. A last bit of foreshadowing comes when Amir remembers the orphanage director telling him how Sohrab was
... inseparable from that thing... He tucks it in the waist of his pants wherever he goes.
It is the slingshot, once Hassan's weapon, and now Sohrab's. And like Hassan had promised to shoot out Assef's eye as a child, Hassan's own child would fulfill his father's promise.