According to Clarisse, what act proves that Montag is not in love?

Clarisse claims to be able to tell that Montag's not in love by the fact that the dandelion's particles don't rub off on his chin. This is an old-fashioned superstition, an old wives' tale which in and of itself proves nothing; and yet, Clarisse is right in her conclusions, even if the method she uses to get at the truth isn't.
For Montag really isn't in love at all. And it's not hard to see why. His wife Mildred is his exact opposite in almost every conceivable respect. Shallow, cold, unfeeling, and completely lacking in intellectual curiosity, she epitomizes everything Montag's come to despise in this dystopian society. As such, Guy and Mildred have no marriage to speak of; they're on completely different wavelengths; in fact, they might as well exist on different planets given how little they share in terms of common interests and affections. That Guy can't even remember how he first came to meet Mildred bears eloquent testimony to the complete lack of love in the Montag marriage.

In Part One, Montag becomes friends with his unique, enthusiastic teenage neighbor, Clarisse McClellan. Clarisse McClellan is unlike anyone Montag has ever met and is an insightful, independent girl, who has an affinity for nature and a curious personality. One day, Montag walks out of his home on a rainy day and runs into Clarisse, who is holding a dandelion in her hand. She then asks Montag if he's heard about rubbing dandelions under his chin before and she proceeds to rub the flower on her chin. She then asks Montag if it rubbed off her chin, which means that she is not in love. Clarisse then rubs the dandelion under Montag's chin and says,

What a shame...You're not in love with anyone.

Montag is initially offended by Clarisse's comment and insists that he is in love. However, Clarisse has revealed the inconvenient truth about Montag's marriage. Montag realizes that Clarisse was right and that he is not in love with Mildred. He cannot even remember where he met Mildred, and his interaction with Clarisse influences him to change the trajectory of his unfulfilling life.