Since the spin axis of a slider is coming towards the batter, you can see the "pole" as a reddish/pinkish spot (depending on the grip). It can vary in size - a tight one is often referred to as a "dime", while a looser one is a "nickel". A dime will break later and harder. Depending on the exact grip and snap when thrown, the spot is usually on the upper face as seen from the batter's box. Because of this, a very tall pitcher can throw a slider that is recognized later by a short batter because of the angle of flight relative to the plate. (I'm 5'7'' and couldn't see a slider spin initially from a pitcher taller than about 6'2" throwing from the old 14" mound) I believe this is why some shorter batters have such trouble seeing the slider (think Jose Lopez). A curve's spin is more perpendicular to its travel and so is seen as a more general pinkish blur. The rate of spin can be judged, and its angle, but if a fastball is expected, adjusting is very difficult. However, a shorter batter with good eyesight can often see that a curve isn't spinning fast enough to break hard and punch it (again think Jose Lopez). Adair's "the Physics of Baseball" and several websites have better graphical explanations, but that's the basics.