Jeff explains to John (and everyone else) how to predict the green speed. Many techniques to use like simply walk on the green. Is it spongy (slow) or firm (fast)? Is the grass dark or shiny? That tells you if the grass is growing toward you (slower, more friction) or away from you (faster, less friction).
Jeff also offers some tips and techniques on how to handle the different speeds once you are able to predict the speed acurately.