((*peeks out from a bush at Redwing*
He's close. pass me the tranquilizers. We may be able to get him to post more around here
LukeKatarn: Much better, the whole thing about the second-most powerful man in the world being in a military unit, when he's supposed to be a polititian, without being recognized, even though he's carrying an ID badge around
*rewinds tape back to factory part*))
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.