Meanwhile the man looked around, and headed for the bar. He ordered a drink while he waited.
Once he was sure no one was following, Lio hid in an alley. He checked his newly acquired goods.
The gems were valuable, though not overly so, he pocketed them. Then he checked the scroll. He shook his head and laughed. Some nonesense about the famous amulet. Seems liket he guy he was trailing had been taken in by that legend. At least he'd be able to sell it to someone who truly believed the legend.
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.