Fortunato doesn't believe that Montresor is a Mason until he shows him the trowel, but this foreshadows why he is carrying that tool
you do not comprehend?
Not I,
How?
It is this
This scene foreshadows what Montresor is going to do next with the trowel. Montresor has the trowel and is casually talking to Fortunato who has no clue that Montresor is about to chain his arms.
The Amontillado!
pass your hand over the wall; you cannot help feeling the nitre. Indeed, it is very damp. Once more let me implore you to return. No? Then I must positively leave you. But i must first render you all the little attention in my power.
This whole scene is both ironic and foreshadows because in the scenes that are ironic you would have to read the whole story to understand the irony. But there is a lot more to dialogue than what you are reading. The foreshadowing is also very well put together because everytime it was foreshadow it was the trowel and he would be sneaky and it would kind of build up suspense for what Montresor was planning.