Just think about it, or how a whiff of Amortentia can clue you in

The one thing out of place this year was that Malfoy and him weren’t butting heads as usual. If anything, Malfoy seemed to be actively trying to avoid Harry for a while now. He missed every single game against Gryffindor, he sat at the back of the class at Potions. He always asked for the most random hours to do his rounds as prefect, which never coincide with his. There was also a lack of hostility towards any of Harry’s friends.

Draco Malfoy seemed to be a shadow of himself. And that bothered Harry.

Or

The thing about Harry was that sometimes he could be a little dense. As Hermione so often reminded him, he was witty and resourceful at a moment's notice or in dire situations, but he could also be painfully slow to notice things that were right under his nose.