What sign does Montresor give Fortunato to prove he is a mason and how does Montresor's proof foreshadow the crime that is about to occur?
As Montresor is leading Fortunato through his family's catacombs, Fortunato takes a drink of De Grâve and makes an obscure gesture with his hands that Montresor does not understand. When Montresor indicates that he is not familiar with Fortunato's hand gesture, Fortunato says that Montresor must not be a Mason (the Masons are an enigmatic fraternal organization). Montresor responds by saying, "Yes, yes" before opening his cloak to reveal a trowel, which is a small tool used by masons to apply and spread mortar.
Montresor's response claiming that he is a Mason is an example of a double entendre, which is a phrase or word with two different interpretations. It can be interpreted as foreshadowing his plans to build a wall around Fortunato using the trowel underneath his cloak. Montresor will bury Fortunato alive in the depths of his family's vaults by building a wall of stone around his enemy, similar to how a stonemason might.
While leading Fortunato deeper into the catacombs, Montresor observes Fortunato making a gesture with a bottle of wine, a gesture Montresor describes as "grotesque." This gesture is a test, one that Fortunato employs to set himself apart from Montresor as a member of the brotherhood of the Masons. When Fortunato reveals the meaning behind the gesture, either Montresor deliberately misunderstands Fortunato's snobbish comments that deny the possibility of Montresor being part of this brotherhood, or Montresor plays dumb, as a way to imbue Fortunato with a false sense of superiority over Montresor.
In either case, Montresor's presentation of a trowel as a proof of his Mason qualifications is evidence of Poe's skill with irony. No matter the reason behind Montresor's claim to the brotherhood of Masons, Montresor's own grotesque and murderous gesture will far outdo Fortunato's.
The sign that Montresor gives Fortunato to prove he is a mason is a trowel, a tool used to smooth and spread mortar between bricks and also a symbol of the masons. The trowel proves to be not only a symbol but an instrument of foreshadowing in that this is the very tool that Montresor uses to lay the bricks that encase Fortunato within his brick tomb once they are inside the catacombs.