Before Daisy explicitly reveals her love for Gatsby, Tom realizes that something is going on between the two of them as the group makes their way to the city. As the group (Daisy, Tom, Gatsby, Nick and Jordan) decide which cars to take, Tom notices glances shared between Daisy and Gatsby. He reveals that to Nick and Jordan that he has just discovered "something"; this something is the affair between Daisy and Gatsby. Later, in the hotel room, Daisy reveals her feelings for Gatsby, yet she cannot renounce her feelings for Tom. Tom, however, refuses to believe that the love between Gatsby and Daisy is true; he exerts quite a bit of energy making Gatsby's shady dealings come to light so that Daisy will forget him (Gatsby).
At the opening of the section in which Tom finds out about the affair, Nick joins Gatsby for lunch at the Buchanans. Tom is affable to Gatsby, and Daisy introduces her daughter Pammy to him. It's very hot and Daisy starts to insist they go to town to escape the heat. At this point she says twice to Gatsby that he always looks so cool. Tom realizes as she states this that Daisy is saying that she loves Gatsby. As the text puts it:
She had told him that she loved him and Tom Buchanan saw.
Nick goes on to tell us that Tom is "astounded." Although the affair has at this point been going on for some time, with Gatsby clearing his house of servants so Daisy can come over in the afternoons, this is the first inkling Tom has had of it.
Daisy wants Tom to know she is in love (or at least thinks she is) with Gatsby and knows how to communicate this to her husband.