((Sorry for not posting, been very sick))
*Matt had overheard Tanara's conversation with Heimdall along with Elaina, and when Elaina leapt to Tanara he ran toward her, once she tackled her he stopped completely, unable to see who was it that he should stop, then started running toward them again once Elaina seemed to have stopped Tanara*
Matt: What happened?
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.