It's in the early pages of Part I when Montag is talking to Clarisse about the dandelions:
"If it rubs off, it means I'm in love. Has it?" He could hardly do anything else but look. "Well?" she said. "You're yellow under there." "Fine! Let's try YOU now." "It won't work for me." "Here." Before he could move she had put the dandelion under his chin. He drew back and she laughed. "Hold still!" She peered under his chin and frowned. "Well?" he said. "What a shame," she said. "You're not in love with anyone." "Yes, I am ! " "It doesn't show." "I am very much in love!" He tried to conjure up a face to fit the words, but there was no face. "I am ! " "Oh please don't look that way." "It's that dandelion," he said. "You've used it all up on yourself. That's why it won't work for me."
Montag doesn't admit to not loving Mildred, but even Clarisse can see that he has no feelings for anyone.
Later, he admits it to himself:
How do you get so empty? he wondered. Who takes it out of you? And that awful flower the other day, the dandelion! It had summed up everything, hadn't it? "What a shame! You're not in love with anyone !" And why not? Well, wasn't there a wall between him and Mildred, when you came down to it? Literally not just one, wall but, so far, three! And expensive, too! And the uncles, the aunts, the cousins, the nieces, the nephews, that lived in those walls, the gibbering pack of tree-apes that said nothing, nothing, nothing and said it loud, loud, loud. He had taken to calling them relatives from the very first.
Whereas Clarisse loves people-watching and nature, Mildred loves the people on TV. Montag begins to see how shallow he and his wife are compared to the passionate young girl.