I have been told that you have already shown aptitude at resisting the Imperius Curse. You will find that similar powers are needed for this... brace yourself, now. Legilimens
This is one of Snape's lines, at the first Oclummency lesson.
How could he know Harry has resisted to Imperius Curse? I mean... Moody (actually, Barty Crouch Jr., a DE) has really used this against Harry, but as it is illegal, would he say to DD that he had done that? I think he would keep it secret! So DD doesn't know it. If DD doesn't know, no one at Hogwarts could know (except the students, of course!)... How does Snape know it? Did Barty Crouch say it to him? Did Snape know Moody was in fact Barty?