Ethan entered his room again and saw that the bed had been made while he was out. This was good. He placed the shotgun between the matresses. Soon after the doctors came in and had a look at him.
"Well you look all right now, but we're going to keep you here overnight, just be sure."
"All right. Can I see my friend now?"
The doctors looked at each other.
"I suppose it can't do much harm. Go ahead. Your friend's up and about roaming the halls."
Ethan went out to look for him.
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.