((GODDAMMIT, I GO AWAY FOR A COUPLE OF WEEKS, YOU GUYS CHANGE THE DESIGN AND POST THREE *** PAGES I HAD TO READ IN ONE NIGHT!
On the other hand, like the ship, Admiral. Reminds me of Hitchhiker's guide to the galaxy))
Matt: See, the thing is, even though you may not have done the kidnapping yourself. You did know about it, and you didn't do anything to stop it in the first place. And you've got enough resources to do it, it's not like you'd be risking doing something impossible. You stood by and did nothing. You consented to it.
*Meanwhile he got an idea. He collected his entire will and force-power, and used it to send a message through the force to four people: Sir-vin, Hal, Tanara and Elaina. the message consisted of a description of their situation, and telling them to tell guy and ask him what to do, and wether they can trust Aren*
((Up to you wether this works RW, I just ask you to remember this is him using every bit of strength and energy he has.))
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.