We know right away that Montresor has vowed revenge against Fortunato because he claims this in the first line. So, this is very blunt and obvious foreshadowing. Montresor claims that it is an "insult" (in addition to the "thousand injuries") that has angered him so much. This might foreshadow a clarification of just what the insult and injuries are, but Montresor never divulges this. Whatever Fortunato has done (or whatever Montresor thinks he has done) must have been damning. In the first paragraph, Montresor says that he will "not only punish, but punish with impunity." In this opening paragraph, Montresor creates the suspense with his incredible hatred of Fortunato. The reader is left wondering how Montresor will exact his revenge.
Fortunato has been at the carnival. He is wearing a striped suit and a jester's hat. He is literally dressed like a "fool." This foreshadows that he will be easy to fool.
Montresor continually warns Fortunato that the vaults are damp and this will affect his cold. Fortunato replies, "I shall not die of a cough." This is merely suggestive. If the cough doesn't kill him, what will?
They take a look at Montresor's family's coat of arms. It is a foot crushing a snake with its fangs in the heel of the foot. The inscription reads, "Nemo me impune lacessit." It means, "no one can harm me unpunished." This is clearly a foreshadowing statement. Fortunato has harmed Montresor in some way and Montresor's motto dictates that he must punish Fortunato. Figuratively, it is Montresor's foot crushing the snake (Fortunato) whose fangs (insult) attempted to injure Montresor.
Fortunato's name is ironic. It means fortunate or blessed. This is foreshadowing using situational irony. His name suggests that he is fortunate, but the opposite turns out to be the case.