Originally posted by Redwing
Hey, you don't need to pay me to do that. I'm posting it so other people can add their own stuff to it
Yeah, but I have no imagination. if you notice most of my stuff is ripped off from other places/people/real life.
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.