((You forgot to mention anything more that Car'nessa is mostly water, like earth. Now get on with the World Bible
Arthur: Get on with it!
Tim: Get on with it!
Crowd: GET ON WITH IT!
God: GET ON WITH IT!))
Matt: Really? Now there's something he forgot to mention when we asked.....
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.