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?

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.