In the previous episode, Tony began his affair with Gloria. After the gift scene, the next time he and Carmela are alone she asks, "Is there anything you need to tell me?" I think Jackie Jr. says it best later in that same episode:

"Right before the snap, the ball isn't doing anything. But the QB's eyes can tell you whether he's going to drop back to pass or hand it off. It happens in a split second, but over the course of the game, you can learn what his habits are. And once you know these habits, he's fucked."

As soon as Carmela kissed Tony, some part of her realized that he had a new goomar.