There are a couple of incidents in this week's chapters (and there will be more in next week's) which make Elinor believe that Marianne and Willoughby are engaged: the gift of the horse ("From that moment she doubted not of their being engaged to each other"), and the gift of a lock of Marianne's hair to Willoughby ("Margaret related something to her the next day, which placed this matter in a still clearer light.").

What I have a hard time getting around is why Elinor did not talk to Marianne about it, ask her if Willoughby had proposed? I can't imagine Marianne would receive a marriage proposal and not shout it to the rooftops - that's what a person of high sensibility would do, is it not? ;-)