Donnie rode back to his hideout, making sure he wasn't followed. Now things would get interesting. He looked over his plans. Now the Destroyers would come after them for revenge, the smaller gangs to gain respect and territory, and probably the Strikers in fear that they'd strike back for abandoning them.
What none of those gangs knew was that the Slickers currently consisted of Donnie and a few close friends, and didn't currently have any territories.
That would soon change. The Slickers had been the most benevolent gang to civilians, and the ones in what used to be their territories remember that. He could count on something the others could not: Support from non-gang members. He took out the slip of paper, and looked at it. He'd call later, for now he needed rest.
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.