How do batters in baseball determine when and where the ball will cross the plate?

I just heard Reggie Jackson say in an interview that batters can tell a slider is being thrown because they can see a red dot formed by the stitches on the ball as it leaves the pitcher’s hand. He also said that you have to be able to do this to last in the majors. I will take his word for this, though it seems quite a feat to me to be able to see a small red dot on a small object 60 feet away.

I always thought that batters adjusted to the pitch as the ball approached. I realize they only have about 2.5 seconds to do this. So my question is, to what extent can a batter adjust the instant that the ball is released and to what extent do they adjust while the ball is in flight?