Originally posted by Deac
[Oops. My Bad. Then again these are evil gods and sith we're talking about and not ones to really play by the rules...Half Life 2 is installing so I'll post later...]
((LOL, I'm installing it too! 24 minutes left.. And Orthos could be covered with with blood from sith slayed by the Aesir.))
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.