If Obi-Wan believes Anakin to be dead when he falls into the lava at the end of ROTS (which he probably will), then how does he know that Anakin is Darth Vader in ANH? More importantly, how is he aware of the existence of Darth Vader? He's been on Tatooine since just after Luke and Leia were born, and he was basically a hermit, keeping away from civilization and news of the Empire's reign. The same goes for Yoda as well. Is it through the Force?