Actually, truth be told, Jane Austen is not a good author to use for trying to find typical literary devices such as metaphor (simile is a type of metaphor), personification, etc. The reason for this is that she was one of the first Realists who wrote as directly and factually as she felt she needed to to meet her desired thematic aims. One can usually find some foreshadowing here and there though and a few allusions scattered about.
It was several days before Willoughby's name was mentioned before Marianne by any of her family; ... but one evening, Mrs. Dashwood, accidentally taking up a volume of Shakespeare, exclaimed,
"We have never finished Hamlet, Marianne; our dear Willoughby went away before we could get through it. We will put it by, that when he comes again...But it may be months, perhaps, before THAT happens."
"Months!" cried Marianne, with strong surprise. "No—nor many weeks." (Chapter 16, V. I)
This artfully crafted passage contains allusion and foreshadowing. Actually, it contains three elements of foreshadowing, a particular Austen authorial device technique and talent. The allusion is a Renaissance one to Shakespeare's famous dramatic tragedy, Hamlet. The allusion comprises several ideas in it; I'll try to paraphrase the essence of the allusion. Bear in mind that the function of an allusion is to convey complex information in one neat image thus you will find that any paraphrase of an allusion will be lengthy compared to the (in this case, single word) allusion.
- Hamlet is a tragedy in which the hero is killed by his enemy's hand while all whom he loves dearest die in the plots that have been laid to snare his enemies and to snare him. The victor stands alone and is one of Hamlet's enemies. In the end, Hamlet receives compassion and mercy for his actions but forgiveness is harder for him to receive. Willoughby is Hamlet and will lay plots that ensnare those he most dearly loves while in the end being snared himself by his enemy. He will beg mercy and compassion, which will be given him, but forgiveness will be harder to ask and attain.
This relates to Sense and Sensibility, and indeed summarizes the upcoming plot (think of occurrences at and after the ball in London), by representing the role Willougby will play in the ensuing drama as it acts itself out in London and near Combe Magna. Thus "Hamlet" represents the first element of foreshadowing, embedded as it is within an allusion.
Mrs. Dashwood provides another foreshadowing element when she says that they will save the play aside for Willoughby's return. This brings up the question of his return in the context of a tragedy (Hamlet) resulting in foreshadowing upcoming tragical events related to Willoughby and the esteemable Miss Grey. Her next statement about months passing before they see him again deepens the foreshadowing by suggesting the upcoming breach between Marianne and Willoughby. Marianne's response foreshadows how she will react to the events being foreshadowed. We feel she will not fare well because it has been impressed upon us in several ways (e.g., characterization, her visit to Mrs. Smith's Allenham) that she does not moderate her feelings or expectations and that she is not prudent.
[Marianne] was ... eager in everything: her sorrows, her joys, could have no moderation. She was generous, amiable, interesting: she was everything but prudent.
Thus here you have one allusion and three foreshadowing elements all tucked precisely together enriching the story for those who know how to analyze them and--more importantly--for those who don't know how.