KICK'EM OUT - SLICE'EM IN

These pictures show the different impact points that the receiver faces on 2 of Roger's second serves.The orange ribbon is the flight of his serve.
The white spot is the point at which Roger hits the ball, the pink where the ball hits the service court, and the yellow the point at which Bjorkman hits the ball.

The upper pix is a kick serve which bounces high and to the left of Bjorkman before he hits it.

The lower is a slice serve which curls into his right side after the bounce. Bjorkman didn't pick up the difference early enough and dumped this return in the net.

How does a returner distinguish the two:
[1] the ball arcs a little more on the kick serve before the bounce
[2] Roger's impact point on the kick is slightly lower [6-8 inches] and to the server's left than the slice. If you stare at the virtual court upto virtual blindness, you can see the difference.