((*Stands in doorway of film development room and store* YOU SHALL NOT PASS!
Anyway I've decided to bring in another odd character to be a hero.))
Name: Seth Greco
Bio: A proffesional thief, he formed a gang which was very succesful. After their last job, he was forced to become a mercenary due to a debt. He found out he was pretty good at it so he kept being one once in a while.
*Greco turned towards Deac and Co.*
Greco: So, why are we here? Did I come into the wrong party again or something?
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.