Elizabeth enjoys talking with Colonel Fitzwilliam. There are indications when someone is interested in another person - the pupils dilate, they turn their head when the person enters the room, they follow with their eyes, they lean forward in conversation. We know there's a lot of energy in Elizabeth's and Colonel Fiztwilliam's conversation:

Elizabeth had never been half so well entertained in that room before; and they conversed with so much spirit and flow...

This may have been all the clues consisted of. And Elizabeth's reactions may have been upped by contrast with the boredom she had come to expect from dining at Rosings.