Looking at the marks left on your club head is a really great way to get information about what exactly happened during impact to cause the golf ball to fly the way it did. Understanding how to diagnose the marks on your club head can then provide you with valuable information on the improvements that are then required in your swing to improve your golf shots.
If you practice hitting shots off a mat at the driving range, have a look at the sole of your golf club. The sole, or underneath of the club head will have been marked when the club head has struck the mat. If you see marks that are at a right angle to the club face then this shows you that the club head was travelling directly along the target line as it struck the golf ball. Therefore, the club face aim was responsible for the golf ball moving the way that it did, so you know you need to improve the club face position through impact. If the ball flew right, the face was open and if the ball flew left, the face was closed so you can now work on achieving a better club face position, where it is aiming at the target, as the club head strikes the ball.
If the sole marks are angled from the toe or far end of the club head towards the heel, or near side of the club head, then this indicates that the club head is travelling across the target line through impact, from the far side, or outside, of the line to the near, or inside, on an out to in swing path. The opposite is true of marks angled from the heel to the toe. You now know that the club head needs to move more directly along the target line as you strike the golf ball and you can begin to achieve working on this.
If you look at the club face, you will see marks left by the ball as it impacts with the club head. The marks should ideally be in the centre of the club face, on the sweet spot where you will obtain maximum accuracy and shot distance. If you notice that the marks are more towards the toe, this would make your shots fly right, as the toe will slow down as it impacts with the ball but the heel will continue at the higher speed and rotate the club face to the right. This is why your shots are missing to the right of the target and you know that you need to achieve a more centred ball strike to improve your accuracy. Try standing closer to the golf ball as this will centre your strike. The converse is true for marks towards the heel, which would produce a closed club face and hit shots to the left of the target. Here, standing further away from the ball would centre the impact more and get you hitting straighter and longer golf shots.
When you are next at the range or practice ground, have a look for where the marks are on your club heads as noticing these will help you to diagnose the issues that you are having with your golf swing and get you hitting more consistent, more accurate and straighter golf shots.