((I don't, Ethan's playin' the part, making it convincing.))
When Ethan left, he went straight to the hotel where he had made a reservation. His reservation had been cancelled, but luckily there was still a room available. He called the hospital and asked it to send the bags over to the hotel, the hospital replied that it'd try to send someone by the next day.
((BTW, shouldn't it become night soon? Or did I miss that?))
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.