Zon looked at her go and went in with the council. They were already arguing over why the humans were attacking. When they saw Zon entering they all shut up.
Zon spoke. "I do not know why the humans are attacking us, but I know part of what they're after. We've found two draconian children, apparently they had ben chased by a blackguilder."
"Who hasn't, these last few days?" Said an elder member mockingly. There were a few chuckles, and Zon grew angry, but still kept calm.
"These children are the dragons that are right now defending our land. The humans are dangerous, we need to stop them now." He said.
"Why shouldn't we just give them the children?" Asked someone else. "If that's who they're after they would make a good sacrifice."
The council erupted into debate, an Zon, horrified at the idea of giving the two children up just for a truce yelled. "EVERYONE SHUT UP!"
The council looked at him, shocked.
"The children weren't here when the attack came, if they're attacking it's for something else. How long do you think a truce will last? Especially if we show such weakness that we give them exactly what they want just for a little peace? They'll keep the peace for a year, maybe two. But humans don't live nearly as long as we do, and they sure as hell don't have patience. They'll invade, confident that we're weak. The only way to stop them is if we stop them now."
"And how are we going to do that? If what you say is true, then we'll need all our agents and soldiers just to protect our borders."
"You won't' need to. I'll go. With Recca."
The council erupted again, how dare this youngster break the council's command that everyone ignore her. They seemed to forget that they had given him permission to go out and find her.
Zon sighed. He wished he could be out in the battle with Recca. The odds there, at least, would be better.
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.