Donnie pocketed the note, and ran out to the roof. He quickly slid down the fire escape. He looked around, there were two motorcycles parked in the alley. He took one and sped off.
As soon as the limos detonated, green smoke erupted from the building. The message was clear: The Green Slickers were back.
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.