There are several ways to determine the number of twists per unit of length. They vary in accuracy and ease of use and those that are more accurate in theory are considerably less practical in practice. In all cases, each method can only measure the specific section of yarn being tested and multiple samples will give a better picture of the overall answer. Methods of interest to handspinners are described below.
Twist angle is the apparent angle of the twisted fibers in the yarn. Measure it with a protractor or a specially marked chart and the axis of the yarn as zero. A small angle is a less twisted yarn and a large angle is more. It can be very difficult to measure the angle of twist for small yarns. There is a trigonometric relationship between the twist angle and twists per unit length based on the diameter and the length of one rotation of the yarn. It is only as accurate as one's measurements of twist angle, twists per unit length and diameter and one's interest in engaging in mathematical theory.
An extensive discussion of twist, twist angle, and twist measurement can be found in Physical Testing of Textiles by B. P. Saville.
Maximum 12 points
Examiner 1 approved of the "mathematical theory" comment, something I included because I think that sometimes people get too worked up about measuring twist. The entire concept of measuring twist is a product of industrial textile production, where it is yet another knob on the machine. It is impossible to do any sort of counting of rotations on a spindle and I completely cannot see an ancient spinner tracing fibers or unwinding yarn to measure the twist. It is either hard or soft, and fairly consistant results can be had by little more than practiced observation.