Originally posted by Redwing
((OOS: But they have the same personality (well, very similar) and the same name
Since it's a new page, I repeat: Jokemaster, where are you?!
((Wow, I've been missed. there's like 5 posts asking where I am/complaining about my not posting/etc.))
*Matt starts recovering from the attack inside the ship. He wakes up slowly.*
Matt *grogilly*: Huh? Did we beat those guys?
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.