"Stun cuff him, Corporal"
The corporal did as Dag ordered him to.
"All right. Now we need to head to Coruscant. Try to rig the ship to send out a signal informing everyone we're friendly. If the aliens have struck elsewhere, they might attack this ship on sight."
"On it." Said the engineer, and he began working on the terminal in the cockpit.
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.