Owen and Beru definitely knew.
"He has too much of his father in him."
"That's what I'm afraid of."
I always figured that Obi-Wan kept up with news around the galaxy by visiting Mos Eisley or talking to other people. It probably didn't take him long to figure out Anakin was Vader. Plus, I remember in ROTS that Obi-Wan watches a video of Palpatine calling him "Lord Vader."