Originally posted by Alkaselzer
Twisted & Evil: How Did Obi-Wan Know?
Everything you want to guess about, right there.
This is way before ANH, but...
Padme:"There is still good in him"
This would ofcourse imply hes still alive, if theres is still some kind of psuedo connection between them. Obi-Wan could've picked up on this and did a check on the state of the empire before he left into isolation on Tattoine. Vader information would've naturally been public as he is one of the top guys in command.