((Viking, try the Vampires thread, 2 blocks down 3rd thread, you can't miss it))
When the first soldier that struck Edwards shield, not counting the one that had managed to impale himself on his spear, Edward quickly thrust the shield up and threw the soldier back, then thrust his spear forward again. He unimpaled the two bodies from his spear.
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.