Jokemaster began doing the job, but quickly grew frustrated with having to go back to areas he believed he was done with after from another angle spotting some trees. Finally he switched the sky to a bright violet, which contrasted nicely with the unfinished trees, allowing him to more efficiently finish his work
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.