The technique used to check a haircut for precision of line and shape is:

The technique known as cross-checking is essential in haircutting for ensuring precision of line and shape. This method involves parting the hair in a different direction than it was cut and assessing the overall shape and length from multiple angles. By cross-checking, a stylist can confirm that both sides of the haircut are even and that the shapes match, thereby enhancing the integrity of the final look.

Cross-checking helps to identify and correct any inconsistencies or irregularities that may not be visible when viewing the hair in the same direction it was cut. It provides a comprehensive evaluation of the haircut, allowing for adjustments as needed to maintain balance and refinement. This technique is particularly important for maintaining the desired shape and flow of the hairstyle.