((well, since that won't work.))
*notices burns are heavier than he thought*
well, this guy won't walk out of here
Comm officer: Sir?
OiC (officer in command) : yes?
Comm officer: we got permission to move in.
OiC: great. Green team move in, Red and Gold Support, Blue team stand by.
*sees the military moving in*
Foley: well, there goes the easy way
*drags Davin to the edge of the stairs and starts signaling the military guys to help him*
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.