((That's right, i think new people come in the beginning of stories, like Catina 8, but in Cantina 9 was the "sequel" to that, and so is this.))
*The Spook was searched for weapons, when they found he had none, the security soldiers told him to wait*
Spook: I'm not going anywhere.
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.