How long has Snape been reading Harry's mind?

[potioncat]

No: HPFGUIDX 134834

 "oiboyz" <oiboyz at h...> wrote:
> snipping<  
>    This is likely to make me re-interpret every Snape/Harry 
interaction 
> in the canon.  If Snape can really read Harry's mind as easily as he 
> did for Sectumsempra... then surely he'll have been doing it for 
years 
> now.  There have been plenty of times when Snape tried to wring 
> confessions out of Harry, and he always seemed to know or suspect 
> whenever Harry was lying.  On the other hand, there were times when 
> Snape definitely didn't get the information he was seeking.  I 
> speculate that he could be under orders from Dumbledore not to use 
his 
> powers against the students; certainly that's one of the first orders 
> I'd give if I had a bad-tempered mind-reader on my staff.

Potioncat:
It has seemed to Harry since the very first book that Snape 
is "reading" his mind. And it seems very clear that Snape is using 
Legilimency in CoS when Mrs. Norris has been petrified. He certainly 
knows that Harry isn't telling the truth.

Now in HBP there is no doubt (to me at least) that Snape has become a 
Legilimens. The real question is, was he a student of Legilimency in 
the earlier books or was he already skilled?

I think in GoF when Snape was threatening to use Veritasarum, he was 
trying to call forward the memories that pertained to stealing 
Polyjuice potion. Sort of a heavy handed method a less skilled wizard 
would use. Which makes sense if he was still learning the technique. In 
CoS, it seemed to me, that he was telling DD that he sensed dishonesty 
in Harry. DD replies, innocent until proven guilty. Now, if you think 
about it, DD has had situations where he's sensed something or flat out 
known something but took no action. There must be something to 
Legilimency that makes it, by itself, unreliable or unacceptable as 
proof.

Potioncat