Originally posted by Scar Da Kookee
((*Posts a friendly reminder that May 20th is my grad date, and from there i have no idea where i would be sent, what ever or not i would have internet access... hard to come by on a ship....*))
((On the other hand you'll be able to watch Episode III on the 19th.....))
Matt *to tank-guy*: Hey, you mind moving your arm a bit? You're holding me at a really awkward position, and it's killing my back.
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.