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.