((OK, new semi-bad guy character))
Name: Adrian Bauer
Bio: An intelligence agent recently promoted to replace Greer. Still unknowing of the politicians plots.
*Adrian sighed, first day on the job. He didn't know what had happened to Greer, he seemed like a good man. His boss had given him free reign. He quickly called one of his officers*
Bauer: Listen, I need you to get a contactor. I just got a hot tip about the Vilnov siblings coming to Corusant. I want to hire them. Send someone in, minunimum armament. Have him with a comlink in case things go to hell. They should be arriving soon.
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.