In "The Tell-Tale Heart," who are the protagonist and the antagonist and how can we tell?
I would say that the narrator is definitely the protagonist but that the antagonist is this, as in most crime stories, is the police. If it were not for the legal authorities, the protagonist could simply murder his uncle, dispose of the body, and enjoy his inheritance of the old man's property. The police are not present throughout most of the story, but it is not necessary for a protagonist or an antagonist to be physically present for his power to be felt. The police do in fact show up and prove that they were always the most serious force to be reckoned with. They will take him into custody and turn him over to others who will eventually try, convict, and execute him.
The protagonist is the main character in a story. Many times it's the "good guy" or the "hero", but the protagonist does not have to be "good". In this story, I would argue that out narrator is the protagonist, so he is the main character in the story.
The antagonist is the character or force that stands in the way of the protagonist getting what s/he wants. So, here you need to define what the narrator wants. If it's simply to be rid of the evil eye, then the antagonist is the old man.