((Well, I must admit I wasn't paying much attention to your PM.
JUST JOOOOOOOOOKING! Really, I thought Agmar was like Hapes, y'know a bunch of planets. And when you said water-based then I thought....))
Matt: My father believed in us looking for information instead of just giving it to us. He never did it with battle stories though, so I'm suprised he did it with this one. Regardless, with that as his 'training' he should have no problem with this.
7 Hours into the battle
*Matt was worrying, the schiltrons were losing numbers rapidly, especially Hal's*
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.