Balance is almost impossible to see when cutting. As the focus is always on the work between the fingers, subtle imbalances on the left and right sides may develop. The mirror is a great tool for detecting imbalances as they develop rather than when they become a significant issue. Standing back from the head allows the eyes to see the outline as a shape instead of a collection of small sections.
As you practice, cut a small portion of the haircut, then stand back and look at it in the mirror rather than looking directly at the hair. This will help you detect imbalances in the haircut because the eyes will process the outline differently in a mirror. If the left or right side of the outline is hanging lower than it should or if the outline is not curving as it should, the mirror will likely expose it. Turn the mannequin head or model while looking in the mirror so you can see the outline of the haircut from different angles.
One of the biggest mistakes that a student makes when practicing is focusing too closely on the point at which they are cutting. When the focus is only on the section of hair being cut, the overall outline of the haircut may become imbalanced. Many students will try to correct this by continuing to cut without stopping to evaluate the outline. The result is that the student will over-cut the hair rather than correct it. Instead, students should stop every few sections and comb the hair down to its natural position before evaluating the outline in the mirror. Only then should they proceed to cut the next few sections.
The following exercise will help students develop the habit of evaluating the outline. Using the back of the head, cut a simple one length trim. Only cut two small sections at a time. Once you have cut those two sections, step back, comb the hair down, and evaluate the outline in the mirror for a few seconds. Repeat this process slowly for fifteen minutes. The objective is not to cut the hair off as fast as possible but to develop the habit of stopping to evaluate the balance of the outline before proceeding.
With time, the eyes will begin to detect imbalance earlier and earlier in the haircut. Small adjustments will become easier because the outline is being evaluated regularly rather than at the end of the haircut. Evaluating the outline in the mirror may seem like an interruption at first, but it will eventually become part of your rhythm of cutting. As you develop your awareness, you will begin to see the haircut become balanced from all angles even before you have completed the final detailing.