Ethan started writing, then soon as he was sure nobody would come in he made a call to the restaurant. He made a reservation for the three of them, describing Carth and jake, and told them that he would be arriving late.
He wrote almost a complete lie, saying that he was driving, saw what looked like a mugging gone wrong, with his friends involved, then fought for them. That his gun was soon knocked out of his hand, adn someone else arrived. He didn't mention any details on Sabrina, other than a description of her footwear. He pressed the 'call nurse' button and asked her to give that report to the cops.
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.