Broken nose. Harry doesn't have any problem puzzling this out. Because he knows the way Draco looks like and how he acts. He also remembers that broken nose well enough.

"Sixth year, is it?" he asks, a hint of a smile gathering on his lips. "I thought you looked a little young." Harry isn't that much older, three years. But he might as well be decades older given what happened during that one particular year.

"You're trying to kill Dumbledore this year. It's not going to work."