Take some chalk (I recommend colored chalk designed for kids to use outdoors) and rub it all over the face of your club (you can also use masking tape or other materials designed for this purpose). Now hit a ball into your net. You should see a nice impact spot somewhere on the club face. If it's in the middle…you've just pulled of a great shot (if the path of the club head was correct - see D-Plane). Pay attention to the blur of the arc down at the bottom of your swing for the other piece of information; if you're swinging inside-out (like you should be most of the time) then you needn't worry over this variable. However, the position of impact on the club face can clue you to the path you're swinging on as well.
More likely, you'll see an impact spot near the toe. Slicers (most golfers who swing outside-in for various reasons--i.e., chicken winging, coming out of posture, etc.) will tend to hit towards the toe, because that's a typical condition of an open club face and outside-in motion. Toe impacts tend to worsen a slice, because the club head will open slightly at impact. Hookers (golfers, not streetwalkers) will tend to hit towards the heel, due to the club face closing through impact and the inside-out motion. Heel impacts serve to close the face even more. Sometimes, the club face is so closed that a shank occurs, and the ball will shoot low and right or run along the ground straight (making you think you topped the ball)!
Where you address the ball on the club face can also affect the mark. It's technically correct to address the ball more towards the toe if you prefer to rest the club on the ground behind the ball (i.e., instead of hovering it), as centrifugal force will pull the club slightly away from your body on the downswing. If you suddenly see your practice shots go low and dramatically right, you're probably shanking off the heel. Use chalk to verify this and then adjust your address position accordingly: Address the ball towards the toe and see what happens to the mark. Note: This assumes you address the ball with the arms naturally hanging; if you vary the extension of your arms towards the ball then this is a variable that will negate learning where you typically impact the club face; work to ensure you address the ball with the same posture and arm position each time!
You may discover that the difference in the club and stroke imparts varying impact points on the club face, depending on the manner in which you stroke the ball. For example, I need to address irons more off the toe to get closer to the center, while woods require me to address closer to the heel (even hovering the club). The reason, I believe, is that irons (shorter clubs) require more of a downward blow, while woods and drivers utilize more of a sweeping action.
However, if you start seeing your ball marks move from the toe towards the heel, it's a GOOD THING. It means that you're learning to keep your extension, swing inside-out, and close the club face…ingredients needed for the draw. As a matter of fact, making changes to achieve an opposite result in golf is usually a good drill (e.g., if you sliced during a round, stop at the range and hit balls until you're hooking it).
Since the club is pulling in towards you, the ball is struck with a glancing blow and on the toe of the club resulting in a slice that doesn't go very far. Does this sound familiar? It is a very common fault, so here is a three-part drill to get you on your way to correcting the problem.
Most golfers think the shank and its violent, clanky offline flight are caused by an open clubface. However that is almost NEVER the case. In fact for most golfers the face is actually closed when the ball makes contact with the neck or hosel. The ball first contacts the rounded, angled portion of the inside of the neck and starts flying offline before any true contact with the flat clubface can be made.
Third, check your clubface. If the golf ball leaves an imprint on the inside part of your clubface (near the hosel), then you have most likely hit the ball from inside-out. If the imprint is on the outside of the clubface (near the toe), then you have most likely hit the ball from outside-in.