There are multiple examples of similes in Edgar Allan Poe's short story The Cask of Amontillado. To begin, one must understand what a simile is so that they can identify them within a text.
A simile is a comparison made between two, typically, unlike things using the words "like" or "as" in the comparison. An example would be: She sings like a nightingale. (A metaphor is very similar, but does not use "like" or "as" in the comparison. Instead the comparison is direct. For example, She is a nightingale.)
Use caution when looking for similes though--just because the word "like" or "as" appears does not make something a simile. Be sure that there is a comparison being made.
As for the text itself, there are only two examples of similes.
"The nitre!" I said: see it increases. It hangs like moss upon the vaults.
Here, the nitre (chemical (KNO3) deposits which collect in caverns) is compared to moss hanging down.
In painting and gemmary, Fortunato, like his countrymen, was a quack.
In this example, Fortunato is compared to his countrymen (very weak simile though, given the comparison made is not necessarily unlike).
As the other educator pointed out, a simile typically compares two unlike things; therefore, comparing two people isn't an incredibly strong simile because two people are not so very different, but Montresor does use a simile to compare Fortunato's current state to the way Montresor was in his own past. He says, "You are rich, respected, admired, beloved; you are happy, as once I was." Montresor is attempting to flatter Fortunato, telling him how much he would be missed if something happened to him because he is so important to the life of the community; Montresor further flatters Fortunato by implying that Montresor, himself, is no longer important in this same way, though he "once was." Therefore, Montresor compares the Fortunato of the present with the Montresor of the past.