This is best done on mid-length putts because the green very rarely has the same slope for more than 20 feet or so.
Find the Fall Line (this is the line where there is no break, up or downhill through the hole). A putt at 90 degrees to the Fall Line will be maximum break from right to left, or left to right, depending on which side of the Fall Line your ball lies. You can learn more about the Fall Line by consulting your copy of The Fundamentals of Putting.
Compare your ball’s position around the hole to that of the Fall Line, knowing that the ball will always break, wanting to follow the Fall Line.
The severity of the break will depend on the position of your ball relative to the Fall Line, and obviously, the overall slope of the green
Practice finding the Fall Line on a green where the hole is cut on a slope, which is the same within a radius of about 10 feet from the hole. Practice putting from 8 equidistant points around the hole, at 45 degree intervals.