Turn your back or side to the sun and check your shadow as you swing. I like using the sunshine and my accompanying shadow for practice, and also when I am playing. I constantly monitor my swing plane to make sure I am staying on balance and keeping the golf club on the proper line to hit the ball toward the target. There really is no mystery. All you have to do is watch your shadow as you swing the club and you will be able to see if you are are swing correctly. The shaft makes a nice arc, a straight line as you swing up on the backswing and down through impact into your follow through. The left arm goes back on one line on the backswing, and the right arm  swings down on the same line through impact and into the follow through.