Originally posted by Scar Da Kookee
((let me understand this
you can buy and install HL2 and not download and install AIM? XP))
((Envisions a battle between gordon free man and the AOL buddy thingy
And yes, that goes for both of us (deac and me, since I no longer have AIM)
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.