He keeps his eyes open when he kisses her. Just in case he says the wrong name. Just in case he forgets, and says 'Draco'.
Ginny certainly wouldn't be too pleased if he did.
When he runs his fingers through her thick red hair, he remembers Draco's silky blonde hair, and how he never really got to touch it properly.
When Slughorn says, "James must be proud," Harry smiles blandly and lies.
"Yeah, I guess love of redheads is just one of those 'Potter things'."
When Remus catches him gazing off into space and asks what he's thinking about, he lies.
"Oh, you know… I'm just thinking of what to get Ginny for her birthday."
When Ron grins and playfully jabs in the ribs with a "Don't go hurting my sister," he lies.
"I won't, Ron."
When Hermione looks carefully at him and asks if he's okay, he lies.
"I'm fine, 'Mione."
When Ginny whispers, "I love you, Harry," he lies.
"I love you, too."
But when he lies awake and really thinks about it until he can't fake it any more, he knows the truth.