Height 5 ft, 9 in
Weight 60 KG
Hair: Brown, short, well kept.
Eyes: Bright blue
Extra: Known as jokemaster, he mysteriously disappeared in the last chapters, but always seems to reappear agian
Jokemaster waited at the same coffee shop Troezen had been in. He saw them greet each other and go in. He ordered another coffee, and something to eat. Finally he stood up, confidently strolled across the street, and knocked on the door.
"C'mon Tara, Jack,Troezen I know you're in there, I'd like to help
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.