Matt: Wow. Couldn't be something simple or riske someone getting hurt, huh?
*After Sir-Vin left, Matt meditated for a bit. Once he finished, he approached the puzzle. *
Well this isn't gonna be easy for me
*he rubbed his hands together and started working on it.*
2 days later
*In the two days since Matt had started the project, he was rarely seen outside his cabin. Inside he had just completed getting the first piece out. He had found out tying his hands behind him had made things easier by stopping him from using them from impulse. The task had gotten easier as he progressed, and he supposed it was set up that way, and his concentration was increasing. He put the first piece aside, and was about to begin working on the second piece when he heard the announcement over the comm. He put the kit and the piece away, carefully making sure none of them would get lost. He quickly dressed himself in green armor, and a vest over it, he supposed that the Aesir would probably consider this formal, so he also put on a formal robe. He then headed quickly to the entrance*
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.