Okay, guys, here's the thing: I don't have time to read all of this thread, so if you could give me the basic story, and what exactly the alternate universe is. Also, maybe post on who is on which side and what types of soldiers are on which side. Meanwhile, my profile.
Name: Santino Sollozzo
Occupation(s): None, under arrest
Physical description: Tall build, dark hair, eyes of unknown color, change depending on both his mood and current lighting. Phenomenon currently being invesetigated by military.
Personality: Removed with a cynical sense of humor.
Hand to hand
Weapons and Armor:Currently only clothes
Possessions:none except clothes
Brief Background: New recruit stationed on board Tho'dane, under arrest for desertion, although this charge is false, he was running away from a jedi who knew he was force-sensitive, but sensed something strange in him. He was caught by the military and set upw tih charges before jedi could explain, the Tho'dane was attacked
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.