Have someone focus on your club as you perform a swing in slow motion. Let them tell you whether the club completes a circular path. If not, tinker with your body position throughout the swing until that person tells you your club has traced a 360-degree circle.
Alignment is about not breaking the swing plane. To visualize this, think of a large pane of sheet glass that is parallel to the angle of your club shaft at set-up. Your swing must remain parallel to this sheet of glass. If you break the swing plane, you have less chance to bring the club face squarely into the golf ball.
A swing path that is on the proper swing plane can still make poor contact with the ball if your wrists don't hinge and release the club at the right moment. This can be demonstrated with another slow-motion swing. Ask someone to watch your club face when the club reaches the impact zone. Have them tell you if your club face remains square during the follow-through. Get a feel for how and when you must break your wrists to keep that club face square.
You predispose yourself to having an out-of-alignment swing if you incorrectly position your body to the ball at set-up. Be sure your club face is squarely up to the ball. The bottom of your club head must stay parallel with the ground. Your toes should remain directly beneath the end of your club shaft. You will be standing at different distances away from the golf ball depending on which club you're using.
Your spine is the rotational axis. It must never move until you complete the swing. Any shifting of the spine throws your club out of alignment and off the swing plane.