Hey wacky the strangest thng, I just came from Timbuktu and it seems you accidently mailed Tarin there, so I brought him back, here you go
*throws tarin at wacky*
By the way, you may not want to go to timbuktu, tarin got you wanted there for somethings you may not want to know about
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.