There is no clear cut answer to this question. If leg length discrepancy does cause symptoms, it is probably back pain. The traditional method is to measure the distance from the anterior superior iliac spine to medial malleolus.
Some experts use the lateral malleolus instead.
Palpate the inferior tip of the malleolus and measure to that point. Measurement of leg length. The superior point of measurement is the inferior border of the anterior superior iliac spine. The inferior point is the distal tip of the medial or the lateral malleolus. Identify the two anterior superior iliac spines and check to see whether a line drawn between them runs at right angles to the longitudinal axis of the examining table.