This technique is employed by many scratch golfers to help determine the direction and amount of break in a putt as well as the general slope of the green.
Determine your 'dominant' eye. With both eyes open make a circle with your thumb and first finger. Hold this circle so you can see a particular object in the near distance, say the flag on the flagpole for instance. Hole your hand steady and close one eye at a time. You dominant eye will be the one where the flag remains in the circle, while the flag will 'pop' out of the circle with the weaker one.
With your weaker eye closed, allow the club to 'hang' freely so the shaft lines up with your ball.
If the hole is to the left of the shaft this means the putt will break left. Hole to the right means the putt will break right. Hole in the center means a straight putt.