((Well he's alternating between 4 people so he's probably only spending a little time on each one. That and how long does it take for someone to swing the saber and lop off a body part. Plus, he is interested in how far Misea has gone into the dark side, its effects, etc. so he would be watching intensly.))
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.