How did he know, exactly? He couldn't pinpoint the moment he knew if he tried. It wasn't entirely a gradual thing either. Perhaps he'd known since the moment they'd met.
"Before Fleur, I'd never loved anyone else. I thought I had been in love before, with this girl I dated at Hogwarts. And I was certainly entranced by several women in Egypt. But Fleur...it was different."
He laughed a little at the fact he was saying all this aloud.
"I knew when I met her that I was in love because nothing else had felt like this. I smiled when I saw her, and it was automatic. I couldn't help myself. I guess I knew when I woke up one day and realized I was the happiest I'd ever been even in the midst of war and all."