What moves do you want to be able to pull off?
1. Send your fighter/bomber ships between 2 enemy capital ships and cause them to shoot each other.
2. Capture enemy ships by disabling their shields and engines and sending boarding craft.
3. Sneaking onto a planet while the space defenses are elsewhere, taking it over, then using it's defenses to blow the enmy spaceships out of the sky.
4. Waging a massive dogfight composed of only TIEs and X-Wings
5. Crush an entire rebel fleet with pure overwhelming force.
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.