((Well, I'm not gonna reveal the contact yet, but I am gonna reveal two possibilities))
Name: Roger Street
Rank: Chief Techie (computer division)
Weapon skills: Light Weapons (blaster pistols) Street can hack into virtually any computer ever made.
Carrying: a blaster pistol, and a datapad
Bio: A techie who specializes in computers, he has acsess to virtually all the imperial computers in the system. He trained with Chavez and has also served under him.
Name: Jim Kint
Rank: Chief engineer
Weapon skills: Hand to hand and light weapons
Carrying: A wrench, and a modified blaster pistol
Bio: An engineer/mechanic. He's in charge of repairing things and has acsess to all the mechanical systems in the system. He also trained with chavez, and served with him on some missions
*Two stormtroopers assigned to find info about who the traitor is find this information. They send it to Commander Ivan*
Vader's wife was very pregnant when she died 19 years ago. All of a sudden a 19 year old who is very strong in the force and has a distinct resembalance to him in his younger days rolls in from Vader's home planet with his old Master Obi-Wan (Who was the ONLY other person preset at the time his wife died. And to boot, the kid's last name is Skywalker.
So in answer to your question, he knew Luke was his son because his name is Darth Vader, and not Darth Retard.
-Forum post on why Vader knew Luke was his son.