Originally posted by Scarface2k2
((It was done person to person, no one else was listening, then the order was then sent directly to the pilots
(( That's impossible, the pilots have to send lots of messages by radio. Then on the matter of someone recoriding it getting a court-martial, first they have to find out who did it, You don't exactly find out much when someone records something and recovers the recording machine))
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.