The sith growled, twisted around and swung. He hit Jedash's saber hard enough to knock it out of his hand, then he kicked Jedash to the other side of the room from the saber. He called Jedash's saber to his hand.
"Not the worst one I've seen. I think I can still make it better, infuse it with the power of the dark side." He laughed, then pocketed it.
"What do you say Jedi? Your lives in exchange for the girl."
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.