Anakin might not be the brightest bulb in the box, but:David Howery said:speaking of the thread title... how exactly did Vader find out that Luke was his son? Haven't seen ROTS yet... did he find out sometime after he became Vader? Or did he somehow guess it all offscreen based on Luke's last name?
1. The name.
2. Feeling that "The Force is strong with this one" from his X-wing (he could probably figure out that it was Luke's ship).
3. Having him turn up at the same time Obi-Wan comes out of hiding.
These kinda add up to some pretty strong indicators that Luke is indeed his son. My guess is that he then uses the Force to make sure once they do meet face to face in ESB.