•Go ahead and take your putting stoke and hit both balls at the same time.
•If the balls move together in a straight line then your club head was square at impact.
•Line up two balls and hit them at the same time.
If one of the balls moved sooner then you know that part of your club head got their sooner.
If the inside ball goes farther then the heel of the putter got there first at impact. This means your clubface was open at impact.
If the outer ball goes farther then the opposite happen and your clubface was closed at impact.
Practice this drill until you are able to hit twenty balls and have them all move at the same time and go the same distance.
You may have to review several fundamentals of your putting stoke to pin point your problem if you are not hitting the balls at the same time.
Revisit your set up, grip, posture, alignment and that your head and lower body are remaining still.
Video on the Two Ball Drill
Source: Golf Tips Magazine- Putt For Perfection http://www.golftipsmag.com/
Remember, if your able to pass this test consistently you will be well on your way better putting and lowering your score.