I have always felt that Montresor reveals his insanity from the very beginning of the story. As soon as he exposes his plan to seek revenge in the opening lines, we get the feeling that he is obsessed to the point of being mentally ill:

THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged; this was a point definitely, settled—but the very definitiveness with which it was resolved precluded the idea of risk. I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.

This is the thinking of a sick mind - planning revenge for an insult! He even tells us he planned this "at length" and didn't merely threaten his friend. Worse, he is going to punish "with impunity." We can actually imagine him smiling his evil smile:

I continued, as was my wont to smile in his face, and he did not perceive that my smile now was at the thought of his immolation. 

As he slyly traps Fortunato into coming with him to taste his wine, we see evidence of his twisted mind in everything he says because we know what he has planned. Finally, when he almost finishes plastering up the wall, he says:

I reapproached the wall; I replied to the yells of him who clamoured. I re-echoed, I aided, I surpassed them in volume and in strength. I did this, and the clamourer grew still.

At this point, we can imagine him brandishing his sword around his head, screaming, yelling, trying to yell louder than the man he is burying alive. ACKGHGGG!!!

You can read the story here on eNotes.