Forward or Backward Pass?

[BktBallRef]

Originally posted by mcrowder
Y'all (Fed folks) have the WING make this decision? Isn't R in the best position to make that call? (And aren't the wings concentrating on a LOT of things other than the location of the QB when he throws it?)

R is on QB's right and behind him, at a 45 degree angle. QB turns and throws to the right sideline.

How can the R possibly know whether the pass has an initial direction of forwards or backwards?