Appearance: Shaggy brown hair, 5' 9'', bright green eyes,
Weapons: Throwing knives, one long knife, bow + arrows
History: Friend of Recca when they met at Archery training. Developed a crush for her and when she was kicked out kept petitioning and arguing to get her back in. The elders finally got tired of him and told him if he could find her they'd consider it, knowing she had gone out to explore the world, hoping he'd spend most of his life finding her.
Zon had been tracking Recca. He seemed to have lost the trail until suddenly he heard a snap. He looked up and saw Recca hanging.
Grinning, he hid, and slowly approached the tree.
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.