I believe HTI (Hemipenal Transillumination Technique) gives an accurate result in these guys, a method pioneered by Danny Brown. As I understand it, the method involves positioning the animal on it's back and positioning a small, focued beam (ie: Mini Maglight) behind the vent/tail base. What your looking for is a "dull redness", red dots or ovals caused by an increase in blood supply or ossification around the hemipenes (please correct me if I'm wrong).
Perhaps, do a google search or, better still, send "geckodan" a PM?