how does ben/obi wan know that it's anakin/vader in the suit in episode 4: a new hope after all he was not there when he got incased in it and he also left him for dead on mustafar in episode 3 :revenge of the sith so for all he knows he could be dead.
Uhhhh......
Have you read DARK LORD, RISE OF DARTH VADER, the novel? if not, it will be pretty easy, it says in the last pages of the book that Obi saw Anakin in a broatcast of the Holonet in his suit and didn't believe it was him, also, he would have known for his pressence in the Force or because the stormtroopers called him Darth Vader
It's pretty simple. Just because Obi-Wan is all alone out there on Tatooine doesn't mean he doesn't know what's going on out in the rest of the galaxy. He had to have heard people talk about and describe Vader, not to mention the fact the he could feel Vader's presence.
Well, Obi is hardly alone on tatooine. It is never said that he stayd in the desert all day and night, and maybe he did go downtown once in a while, colleting information and enjoying a much needed twi'lek lap dance.
Its difficulty to not hear about someone whos the sidekick of the galactic Emperor... Yes, even in the exile.
I'd go with Obi Wan knowing through the Force that Vader is actually Anakin. Vader was able to work out that Luke had a sister, Leia, so it's safe to assume that Jedi, orthe Force attuned, are able to make large jumps in logic safely, or at least are able to make educated guesses.
^Couldn't have put it better myself.
Yeah it would be pretty suprising if Obi-Wan didnt know if it was Anakin in the suit.
Ofcourse thhey would have sensed each other from the bond that had connected them whilst Master and Apprentice.