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.
Chapter VII includes the "show down" scene between Tom and Gatsby in the hotel room in New York City. Gatsby confronts Tom and tells him that he and Daisy have loved each other for the last five years since they first met. Tom rejects violently the idea that his wife loves Gatsby:
You're crazy! . . . I can't speak about what happened five years ago because I didn't know Daisy then . . . . But all the rest of that's a God Damned lie. Daisy loved me when she married me and she loves me now.
After the violent argument continues between Tom and Gatsby, Daisy confirms that she loves Gatsby when she says, "I love you now," but she acknowledges that she once loved Tom, too.
Tom's ego can't accept that Daisy feels any real love for Gatsby. He sends her back to East Egg with Gatsby:
Go on [he tells Daisy]. He won't annoy you. I think he realizes that his presumptuous little flirtation is over.
Tom refuses to believe, simply can't believe, that Daisy could love anyone but himself. The very idea is an affront to his basic arrogance and sense of superiority and security.
Gatsby, Jordan, Nick, Tom, and Daisy are having lunch one hot afternoon at Tom and Daisy's mansion. Daisy and Gatsby are having a hard time not showing their affections for each other. Eventually, Daisy asks Gatsby (and the others, by default) if they want to go into the city. Gatsby stares back at Daisy with such passion that Tom realizes his wife is up to something with Gatsby. He doesn't say anything, then, and he has no real proof at this point, but in his heart he knows something is going on.
He doesn't get proof until later that day when Gatsby, in the hotel room with the others, confronts Tom with the fact that Daisy is now in love with him and, in fact, never loved Tom at all. That's when all hell breaks loose.