When ethan arrived at the room a doctor was there waiting for him.
"Some police agents want to question you, so wait in your room for them."
"All right." Said ethan, feeling suspicious but knowing it'd be worse for him if he ran. He went into the room and immidiately grabbed the phone and dialed Carth's room. When they picked up he spoke quickly.
"Hey I'm getting held up here for some questions. You guys go ahead without me, You know the Grotto restaurant on Remington and Main? I'll meet you there."
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.