"HOLD THE WALL!" Yelled Edward, he saw the man coming towards him, ducked and used the shield, and the man's own speed to flip him over with the shield. The fall killed him, or at least knocked him out long enough for the fire to killh im.
"YOU!" Edward pointed at one of the javeliners who seemed to have run out "GRAB THAT SHIELD AND SPEAR" he pointed at a dead roman auxila body. "AND COVER THE DAMN BREACH. DRAFT ANYONE YOU NEED TO FOR THE JOB, JUST MAKE SURE THE WALL DOESN'T BUCKLE IN!"
Edward hit a man on the head with his spear by swinging it sideways, killing him.
"Damn it. Just hold out a little longer....."
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.