Name: Matthew Mason
Bio: Expert in weapons and science, Matt joined the Ghostbusters after talking to a couple of the members from the second generation in class. Right after finishing school he joined the third generation.
Matt walked in and saw Tepe and another one of the members on the sofa.
"Hey kid, stop slacking off, the rest of the team will be here soon. Help me make some coffee for them."
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.