((Deac, if this disrupts with whatever you have planned, tell me and we can either change this, or just make it that another group is trying to kill Deac))
*Greer and Gilan hear the newscast*
Greer: Same guys that tried to destroy corusant are killing most of the military leaders. This was found on some of their computers, read eight lines down.
It doesn't look as if though Starkiller will survive, but I want your permission to send a force and make sure.
Greer: Starkiller was the word that got them caught, they didn't think I'd be prying into their computers
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.