Ethan still had some things to do. He called the restaurant and made two reservations. One for him and the other for Carth and Jake. He went out to an office suply store and bought a pen and a notepad. He then took a taxi to the restaurant, and sat in his one person table. He spoke with the manager, and asked him to seat Jake and Carth in the next table.
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.