When I was 17...i thought i'd never be 18. now i'm 20. XD
Battle is a pure form of expression. It is heart and discipline, reduced to movement and motion. In battle, the words are swept away, giving way to actions-- mercy, sacrifice, anger, fear. These are pure moments of expression.
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.
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