"Well, I'd just arrived here to see my friend, I'm driving to the hotel, and who do I see but him getting mugged, or attacked by a gang. So I stopped, and helped him out. " He shrugged. He knew he was lying, but he had gotten pretty good at it. Plus if he told them they were attacked by vampires they'd have him commited.
"They were probably on drugs or something, they seemed to withstand nearly anything. I read somwhere that some of these drugs can numb a person so that he can be shot 3 times and still keep on."
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.