Ethan took another note and scribbled quickly on his knee.
"Ok, but I gotta eat. Make this look good."
He ordered some raviolies
The two cops had accepted the meal, but remained vigialnt. They stayed parked outside the hotel, waiting for Ethan to make a move. They looked at their clock, and got suspicious.
"He should be doing something by now. I'm going in to check on him."
"I told you, he's probably tired. But if you want to go, go."
The cop went out, and asked the clerk if he had seen Ethan. The clerk called his room, and no answer. He shrugged.
"Do you want to check his room?"
"No." He didn't want Ethan more suspcious.
"Well, I could send someone to check if he's in."
Soon after he got back in the car, furious.
"He gave us the slip, he must've gone out while they were giving us the food. "
They called in with HQ.
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.