This simple but effective drill will help you with a number of your different putting fundamentals.
It so simple to set up, just simply tie a length of string to two pens or pencils and spike it into the green either side of the hole. Try to find a flat part of the green where you have a putt with no break.
The idea then is to try and roll the ball down the string line into the hole. If there is a swing path error in your stroke it will become clear as the putter head will deviate either side of the string during your stroke. It is also a great drill to help you with your alignment and also helps to get your eyes over the ball. Give it a try next time you practice.