"But this is important."
"Something you have to say is more important than the Ministry of Magic, Potter?"
"Look," said Harry, throwing caution to the winds, "Professor—it's about the Sorcerer's Stone—"
Whatever Professor McGonagall had expected, it wasn't that. The books she was carrying tumbled out of her arms, but she didn't pick them up.
"How do you know—?" she spluttered.
(Harry Potter and the Sorcerer's Stone)
It seems that the highlighted part has the perfect tense meaning –– for “the perfect is a kind of past tense (SGEL,p78, 86)” and Harry’s knowing is preceding the speech time. He’ve known the fact and now he speaks out what he’s known. And ‘how do you know’ seems to contain all this. Can this be right, and the present also have the perfect tense meaning?