Hit 2 balls.

Set up 2 golf balls side by side and then walk in to your putting set up with the putter head behind both balls as if you are going to hit them both at the same time.  Pick a distance it can be long or short and create a putting stroke relative to that distance.  Your goal is to hit both balls at once and see if the balls travel a similar distance.  Watch what happens.  Does one travel further than the other, if so which one.  If the ball that was closest to the heel of your putter face travelled further then your putter face was open at impact imparting side spin and backspin on the ball.  If the ball closest to the toe of your putter face travelled further then your putter face was slightly closed at impact.  This would be the preferred outcome.  It indicates that you are creating side and top spin on the ball which will keep the ball on line and rolling more smoothly.

Ideally the balls would roll a similar distance for long and short putts confirming that your putter face is neutral to the path.

Try this drill for a variety of length putts; the outcome could be different from one length to another.