When they arrived at the office, other than the CIA agents there was also another young man there. He was Jokemaster from in-game, he looked almost exactly alike, except that his hair was combed neatly, and he wore a three piece suit.
((bTW, what the hell is going on? I'm assuming they're being recruited but for what?))
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.