Originally posted by Curt-Man
sorry, thats kind of why i shot him insted of chasing him down and biting him and jokemaster, huh? i don't get what you just said.
I was confirming that you and I are cooperating on my char's backstory, and the rest was from the quote.
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.