"How'd you guess?" Asked Zon. "They're 'debating the issue'. As if there's any question to what should be done."
"The boy is right." Said Gendart reluctantly. "We can't afford to show weakness against the humans."
Another councill member scoffed. "And you're the most objective person in this." He said sarcastically.
Gendart stood up, angry. "Who was it who supported the council's desicion to banish Recca, even over my son's objections? Who refused his requests to void the exile? Your memory seems to be a fluid thing, Elur."
Elur sat down, angry at having be embarassed. The council almost immediately fell back to a shouting match.
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.