((Don't get too used to it ))
Tired of the critisisms Zon shouted again.
"I DON'T CARE WHAT YOU THINK ABOUT HER!" he yelled at them again. "She's out there, fighting for the city, for you, even after you turned your backs on her. She's risking her life while you're here just stroking each other's ego."
The council now looked positively ready to order Zon hanged.
"Now, do you want to keep acting like cowards, or do you want to stand up to the humans? Should the elves now be known as the kind that gave up to the humans after one battle. One battle!"
"They have blackguilders." Said one of the elders. "We can't fight against an army of blackguilders! Nobody can"
"You won't need to. If we strike inside their own territory, they'll send the blackguilders there, to hunt for us. And we won't make an easy chase."
"Two elves alone against blackguilders? Impossible."
"Two elves, two draconians and a wizard." Shot back Zon. "And anybody else who volunteers."
The council stayed silent. Finally one of them spoke.
"We'll debate this for now. Come back after the battle to see if we have an answer."
"It may be too late by then!"
"This is not a decision to be made lightly, young man. " Said one of the eldest members. "Patience is something you should learn."
Frustrated Zon finally left muttering curses.
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.