Pride and Prejudice Volume 3, Chapter 16

Darcy comes with Bingley to Longbourn a few days later, and he and Elizabeth are left alone to walk around as Jane and Bingley pair off and Kitty goes to visit the Lucases. Elizabeth uses the time alone to thank Darcy for what he did for Lydia. He is surprised that she knows because he didn't want her to feel uncomfortable or obligated to him for anything. He tells her that he did it as much out of guilt over not revealing Wickham as to gain Elizabeth's favor. Then he says, "You are too generous to trifle with me. If your feelings are still what they were last April, tell me so at once. My affections and wishes are unchanged, but one word from you will silence me on this subject for ever." Volume 3, Chapter 16, pg. 252

Elizabeth tells him that her feelings for him have changed, and as they walk, they talk of how they've finally come together. Lady De Bourgh repeated to Darcy part of her conversation with Elizabeth, and he knew Elizabeth well enough to know that if she'd not cared for him, she would have told Lady De Bourgh that they weren't engaged from the very beginning.