Edward grabbed the money and went back to his tent. He gathered the men that were there. They grumbled a bit, but went along.They quickly reached the store, not wanting to waste any time, and Edward was anxious to see how Erin was doing. He walked into one of the stores and they started grabbing the food and other supplies they needed
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.