Originally posted by Hiroki
(( Hmm, well, things are going well too, I agree. I'll make sure every character gets a fair share of the action. But for now, we do need a focus for the story. I like JM's character of Edward, and since he is with the Romans, the story is primarily following him at this point. ))
((*does victory jig*))
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.