Name: Ed Foley
Bio: Used to be a Special Operations solder with a reputation of disregarding orders to accomplish the misson. Was on a mission when his whole squad was infected. All joined The Forgotten Angels except him. He is in early stages of mutation, and is searching for an answear to his questions and for the elimination of the Forgotten Angels who he suspects is responsible for the death of his mentor/boss. There are no visible signs that he is a demon yet, but the goverment thinks he has joined the forgotten angels and is trying to find him. He joined forces with Davin because he believed that Davin really was trying to stop the Forgotten Angels who framed him.
Mutations: increased Agility and hearing. He is jumping higher every day. He can also make himself look younger or older at will
*Foley shot the acolyte that got in Davin's way*
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.