Eh, famous quotes from exam week. All of them real
My bud *shaking random student yelling hysterically *:WE'RE ALL GONNA FLUNK! FLUNK I TELL YOU!
Exam proctor: All right, you're going to take this test, you're gonna flunk, if you finish early just sit quietly and think happy thoughts. If you have a question raise your hand and I will do my utmost to confuse you.
*after final exam is over*
Bud: THUS ENDS THE FLUNKING DISASTER OF EPIC PROPORTIONS. ENJOY THE FIRST WEEK OF HOLIDAYS, 'CAUSE WHEN REPORT CARDS ARRIVE, WE'RE ALL GONNA DIE!
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.