Reading putting greens is one of the hardest skills in golf to master
Even if you have perfected your putting stroke and distance control, you will end up missing the target most of the time if you are having problems reading greens. The ability to effectively read greens is more of an art then a science. Sure, it involves the laws of physics, but without instruments available to take accurate measurements, we're back to the art of guessing.
Reading putting greens can be broken up into the following components:
Slope of the Green
During your approach survey the entire area of the green along with the immediate surrounding areas. This perspective allows you get a great idea on the general slope of the green. Take note of what the overall contours are like. Is the green basically flat or does it have ridges, valleys or multiple tiers? Once on the green, you can identify the more subtle contours of the green itself.