48) Every time we get into an OOC discussion in the story thread (drink twice if it involves Scar, BD or me)
49) Every time BD and Scar start arguing over what one of their char's did
50) Every time Cracken disappears for more than a week.
HA! BEAT YOU ALL TO NUMBER 50!
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.