Although Willoughby left his card, what is proof to Elinor that he may have changed his mind about Marianne?

Elinor still has not managed to find out if Marianne and Willoughby are engaged; she is much disturbed when Colonel Brandon approaches her and tells her that their engagement is widely known throughout town, though Elinor does not know of it. Elinor says that she is sure of their mutual affections, but when Colonel Brandon says that Willoughby should try to deserve Marianne, Elinor's doubts about Willoughby's character and intentions resurface. Marianne and Elinor are obliged to accompany Lady Middleton to a party, though Marianne clearly has no heart for it. At the party, Elinor and Marianne see Willoughby; Marianne approaches him, although he turns and addresses Elinor instead, trying to avoid Marianne.