How did Dumbledore know what Ron saw in the Mirror of Erised?

Question

In the Philosopher's Stone (Sorcerer's Stone in the US), Dumbledore says:

"And it showed your friend Ron himself as head boy."

About what Ron sees in the Mirror of Erised.

However, when Dumbledore looks into the mirror he sees what he wants (which Dumbledore says is a pair of socks), not what Ron wants.

How does he know what Ron want, without having interrogated him? As far as I remember, Harry has to tell Dumbledore what he saw. Likewise Voldemort can't see what Harry saw at the end of the book.

Answer 1

Dumbledore was in the room when Ron described what he saw in the Mirror of Erised.

When Harry takes Ron to see the Mirror of Erised Ron says what he's seeing out loud:

Ron, though, was staring transfixed at his image.

"Look at me!" he said.

"Can you see all your family standing around you?"

"No — I'm alone — but I'm different — I look older — and I'm Head Boy!

The two weren't exactly being quiet about it either:

A sudden noise outside the corridor put an end to their discussion. They hadn't realised how loud they had been talking

Coupled with Dumbledore's answer to Harry when asked the same question:

"And it showed your friend Ron himself as head boy."

"How did you know —?"

"I don't need a cloak to become invisible," said Dumbledore gently.

_{Harry Potter and the Philosopher's Stone Chapter 12: "The Mirror of Erised"}

leads me to believe that Dumbledore knew what Ron had seen because an invisible Dumbledore was in the room with them.