How did Montresor know that his house would be empty in "The Cask of Amontillado"?
Montresor knew the house would be empty because he told the servants to be home.
Montresor is an expert in reverse psychology. He knows that if he tells his servants that he will not be home, but orders them to stay there, they will all take off. Who wants to be home during carnival when the boss is away?
Montresor is trying to give himself an alibi. He tells her servants that he would not be home until morning to make sure that they all leave his house.
There were no attendants at home; they had absconded to make merry in honour of the time. I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned.
If they are questioned, they will tell the police that they were at the house all night for fear of losing their jobs. Thus Montresor is not there, and the servants are not there, but if anyone would ask they will all say that they were there. No one will know where he actually was.
If Montresor returns with a trowel and the ingredients to make a brick wall, smelling of nitre from the catacombs, no one will be there to see it. His servants are all out playing while he kills a man.
Montresor is a very clever man, and he plans the murder out to the most minute of details. There will be no one to see him out with Fortunato, and even if they did they may not recognize him because of the costume. Also, many people will be drunk. There will be no one to vouch for the fact that Montresor was not home or describe the condition he was in when he returned.