"Wait for it..."
As the last of the aliens move their attention from the bridge, Dag opened the door. The mandalorians rushed through, a barrier of fire in front of them.
The aliens, stuck between an oncoming wall of fire and a jedi, began trying to retreat. As they got closer, the mandalorians switched to melee weapons to avoid friendly fire.
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.