At what point did Tom realize that Daisy loved Gatsby?

Tom knew that his wife was in love with Gatsby by the way she looked at him. He knew because that was the same way she used to look at him (Tom).

Their eyes met, and they stared together at each other, alone in space. With an effort she glanced down at the table.
“You always look so cool,” she repeated.
She had told him that she loved him, and Tom Buchanan saw. He was astounded. His mouth opened a little, and he looked at Gatsby, and then back at Daisy as if he had just recognized her as some one he knew a long time ago.