When the ball is below your feet adjust for the change in your arc and shift in your weight
Grain is a very difficult concept and there is no way to perfectly categorize its effects on your putts, experience and feel for grain takes time to develop, but by using this trick to see the grain you can practice on it and analyze how it effects your ball to get a better understanding
Make more of your mid-range putts by practicing hitting the putts past the hole with this simple drill, giving you a better chance of making putts because they will all reach the hole