Caer soon reached a small inn, and went in through the back door. It was a dark room with a table in the middle.
He collapsed on the table and removed his shirt. There was a nasty cut on his right arm. Soon enough, the innkeeper arrived. He saw Caer.
"Caer! " He exclaimed. "Give some warning to an old man." He approached. "You got hurt."
Caer nodded grimly. "There were some wolfriders. They began to attack a man, an elf, and a halfling."
The innkeeper sighed. "When are you going to give up this vendetta?"
"When I go to my grave." Replied Caer. "Are you going to help me or not? I gave my herbs to the elf. He got shot."
The innkeeper expertly began tending to Caer's wounds.
"And, as always, you refused to stay with them." Said the innkeeper.
"They can take care of themselves." Replied Caer. "And I saw more elves arrive after I left. "
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.