((I meant that so far in this thread, two people have had to beg to be able to stay onboard. It kinda reminded me of the description a friend gave me of Survivor, never watched it myself))
*Matt took a drink from his canteen, which he had taken with him when he first wen to train*
Matt: Guess so. *He girnned* Maybe pretty soon I'll be able to beat you. Or at least last more than 3 moves.
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.