The most reproducible method.

Heaver C, et al. S
Hip Int. 2013 Aug 9;23(4):391-394. doi: 10.5301/hipint.5000042. [Epub ahead of print]

Abstract
Leg length inequality following total hip replacement remains common. In an effort to reduce this occurrence, surgeons undertake pre-operative measurements, templating and use various forms of intraoperative measurements, including computer navigation. This study aims to delineate which measurement technique is most appropriate for measuring leg length inequality from a pelvic radiograph. Three observers took a total of 9600 measurements from 100 pelvic radiographs. Four lines were constructed on each of the radiographs, bisecting the acetabular teardrops, ischial spines, inferior sacroiliac joint and inferior obturator foramen. Measurements were taken from these lines to the most prominent medial point on the lesser trochanter and to the tip of the greater trochanter. The effect of pelvic positioning was also assessed using radiographs of a synthetic pelvis and femur. Intra-observer and inter-observer variability were calculated. Measuring from the inferior aspect of the ischial tuberosities to the most prominent medial point on the lesser trochanter appears to be the best method for measuring LLD however large error margin still exist, even when corrected for magnification errors.

PubMed Abstract:  http://www.ncbi.nlm.nih.gov/m/pubmed/23934902/