In my opinion, the clearest evidence that Montresor has committed the perfect crime is that he has not been prosecuted for the crime of killing Fortunato. The story is narrated by Montresor, but not the Montresor of the time of the story. Instead, the Montresor who narrates the story is looking back at the time that he killed Fortunato. It is decades later when he is telling the story. If he has not been caught and punished after all these years, he must have committed the perfect crime.
In the last lines of the story, Montresor reveals that he has gotten away with the crime for something like 50 years:
Against the new masonry I re-erected the old rampart of bones. For the half of a century no mortal has disturbed them. IN PACE REQUIESCAT!