With this technique, an ultrsound is used to safely peer inside the Gila to look for either testes or ovaries. This technique is best performed during the winter or spring when a females ovarian follicles are under development. In the past, many methods were espoused.
Some people use body form to differentiate the sexes.
Males reportedly having broader heads and stouter builds. Females reportedly having smaller heads, more elongated forms with bodies tending towards being pear shaped. Please note that although some animals accurately reflect these morphological differences, others are not easily sexed by form alone. At best it is an inconsistent method. I have radiographed several of my Gilas and have not been able to reproduce their results.