In "The Adventure of the Speckled Band," an example of foreshadowing would be the clue that Dr. Roylott has someone from his former residence of India who sends animals native to that land to him in England.
The concentration is on the cheetah and the baboon, giving the reader no reason to suspect either of those animals as they would be unable to get into a room through window or door. This is foreshadowing because it explains the presence of the snake, poisonous, that English doctor's would not know to suspect in a death (actually, murder) at the doctor's home.
It is not possible to give page or line numbers as there is no way to know from what source your version of the story has come. I read the story online, in any event, and there are no page or line numbers.