In Chapter Seven of "The Great Gatsby" as Nick and Jordan and Gatsby lunch at the Buchanan home, there is a phone call for Tom, who takes the call in the hallway of his home. While he is on the phone, Jordan whispers, "The rumor is...that that's Tom's girl on the telephone." But, the group hears Tom refuse to sell a car, supposedly to Myrtle's husband. Tom flings open the door, calling to Gatsby, but he sees Nick, acknowledges him and again leaves the room. When her husband does so, Daisy goes over to Gatsby, "and pulled his face down kissing him on the mouth."
'You know I love you,' she murmured.
Daisy looks around "doubtfully" and sit downs "guiltily" on the couch when Jordan makes a comment. After they have lunch, Tom invites Gatsby outside, showing him a sailboat and the stables. After this, Daisy asks "Who wants to go to town?" Gatsby's eyes "floated toward her...'You always look so cool, 'she repeated."
After this look, Tom realizes the truth of what he has seen from the hall. For, Nick narrates,
"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 someone he knew a long time ago.
When Daisy again speaks to Gatsby, Tom interrupts,
'All right,' broke in Tom quickly, 'I'm perfectly willing to go to town. Come on--we're all going to town.'
He got up, his eyes still flashing between Gatsby and his wife. No one moved.
'Come on!' His temper cracked a little. 'What's the matter anyhow? If we're going to town let's start.'
His hand, trembling with his effort at self control, bore to his lips the last of his glass of ale.
Before they drive to the Plaza Hotel where Gatsby confronts Tom, Tom Buchanan clearly has discovered the affair between Daisy and Gatsby and realizes the seriousness of it.