It’s not always possible to get to the course to play or practice so working in front of a full length mirror at home or the range enables you to check what you feel you are doing and what is actually happening are the same thing. You can use a mirror to practice many drill and to check how you align and position the body. By using a mirror you can make sure your angles are in order so that your body is geared to make a balanced swing.
Looking down the target line, you can check the quality of your alignment, and monitor the path and plane of your swing as you run through some of the key positions in the backswing: moveaway, setting of the wrists, at the top, starting down and so on.
I would also encourage you to swing towards the mirror, to see how the body unwinds through the impact area. Coming into the ball you want to see the left side of your body turning out of the way as the hips, chest and shoulders open up to target, clearing the way for the hands and arms to release the club through the ball. Look for those images in the mirror and feel for your body works as you unwind up to a full finish.
In the final position the hands should be seen to arrive comfortably behind the head, the shoulders should be fairly level and your knees nicely together with your pressure solidly in the left foot, balanced on the upturned toe of the right foot.
Practice these feels and positions until your swing meets these criteria!!!