How does Snape know?

[Hannah]

No: HPFGUIDX 113532

How did Snape know Harry could resist Imperius?  If C!M was working 
> as Moody under DD's instructions, he could have reported it DD.  
But why would C!M tell DD?  And why didn't LV know that Harry could 
> resist it? Do you think Crouch!Moody forgot to tell him?  
> 
> So, do you think DD told Snape?  I wouldn't think C!M did.  They 
> didn't seem the type to sit around and discuss students with each 
> other. Do you think Snape learned about it from a DE contact who 
> told  him about the graveyard events? Or, was Snape there afterall?
> 
Hannah: I don't think Snape could have been in the graveyard that 
night, simply because he can't disapparate/ apparate from Hogwarts.  
He wouldn't have had time to get out of the grounds without being 
noticed, and to get back so fast afterwards.  His absence ties in 
with LV's comments about the missing DE's, and DD apparently asking 
him if he's prepared to return to spying and wishing him luck later 
that evening.

I'm guessing either Crouch!Moody mentioned it in the staffroom (it 
would be in character for Moody) or that DD found out somehow (he 
supposedly knows everything that goes on, and keeps an especially 
close eye on Harry), and gave Snape this information when he asked 
him to teach Harry occlumency.  

As for Crouch!Moody not telling LV, I doubt they had a lot of 
contact, they'd have had to be pretty careful, being right under 
DD's nose.  When they got in touch, they probably discussed the 
details of the plan and not much else.  I doubt Crouch considered it 
even relevant to LV that Potter could resist the curse.

Hannah