Did Dumbledore know that Fake Moody = Crouch?

[Rehan Shamim <rshamim@princeton.edu>]

No: HPFGUIDX 48633

Hello everyone,

After lurking on this list for six (yes, six!) months, I have 
finally decided that the time was right for me to make my first 
post.  Yessss!!!!

My question is whether Dumbledore knew immediately that Fake Moody 
was actually Barty Crouch, Jr. after he stormed into the office and 
stunned him.  He recognizes that this man isn't really Alastor 
Moody, as he tells Harry that the real Moody would have never taken 
him out of Dumbledore's sight.  However, he also seems to 
immediately realize that this man is Crouch Jr.  Soon after stunning 
him, he gives the following instructions to Snape:

"Severus, please fetch me the strongest Truth Potion you possess, 
and then go down to the kitchens and bring up the House-Elf called 
Winky." (GoF, Chapter 35, "Veritaserum," Page 680 of original US 
Edition)

I think it is pretty obvious that Dumbledore would not send for 
Winky if the impostor was just some random DE.  At the point that he 
sent for Winky, he must have known that it was Crouch.  This means 
one of two things.  

Choice A: Dumbledore knew the whole year that Crouch had infiltrated 
Hogwarts, although he may not have known that he was disguised as 
Moody.  For example, he may have thought that he was posing as a 
student or one of the people there specially for the Triwizard 
Tournament (like Bagman or someone).  Once he figured that the 
impostor was going around as Moody, he captured him, already knowing 
that it was really Crouch.  

Choice B: Dumbledore did not know there was a spy at Hogwarts.  He 
may have been suspicious of some people, but not to the point where 
he would do anything about it.  After the Tournament, when Moody 
takes Harry away, he realizes that Moody is the traitor.  In the 
five or ten minutes between his realization that Moody is the 
traitor and his Stupefy!ing of Moody, Dumbledore makes a few 
conclusions.  First, he believes that this man isn't the real 
Alastor Moody.  Probably a safe assumption, as Dumbledore knows that 
the real Moody would never directly disobey him in such an important 
time.  Second, he figures that the spy is none other than Barty 
Crouch.  How does he come up with this idea?  Is he really that 
sharp that he could accurately predict that the man lying at his 
feet was a DE who had supposedly been dead for quite some time?  

Now, I am pretty sure that Dumbledore wouldn't let an unrepentant DE 
roam the castle, so in my opinion, Choice A is pretty much out.  If 
any of you would like to disagree, then please do so.  The question, 
then, that still remains is how exactly did Dumbledore come up with 
the idea of it being Crouch so quickly?  What could he have known 
beforehand that would lead him to this assumption?  Please, try to 
answer this, as I am totally confuzzled!  

Rehan (a guys' name, for any of you who are unsure :-)  )