*A young man crashed straight into RH Irvine*
Name: Matt Marsh
Bio: His parents worked in the mine all their lives, and so has Matt. His parents died when he was 10, and he's been taking care of himself ever since. He's force-sensitive, but no one has ever noticed.
*The young man stands up and apologizes. He notices RH Irvine's saber.*
Matt: You're a jedi?! Can you get me out of here?
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.