Crouch down and take a look in line with the hole, from directly behind your ball. You will be able to tell which way the putt will break, and the severity of the slope will tell you how much it will break. The easiest way to tell which way a green will break is in a practice round (not a competition one) hit numerous putts to areas where a pin may be cut and remember how the ball reacts.
Always watch out for the contour of the green and the ball always breaks towards the sea. You cannot really be taught how to read greens, you need to develop it yourself. The more you play the better you get at it.
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