Sex determination in reptiles is important for implementing appropriate husbandry, successful pairing for breeding programs, and reproductive medical care. In reptile species that are not sexually dimorphic, accuracy in sex determination is dependent upon methods other than physical observation. Preferred sexing techniques should be quick, noninvasive, and reliable, and they should be able to be performed by veterinarians with a broad range of reptile experience. Vast anatomical differences among reptiles make the efficacy of these techniques highly variable between species. This study evaluated hemipenis contrast radiography and computed tomography techniques, previously described in other Heloderma species, to determine reliability in sexing 23 Gila monsters (Heloderma suspectum). The results were compared to the sex determined by coelomic ultrasound, which indicated that there were 8 females and 15 males. Identification of hemipenes in males was shown in 1/15 (7%) by contrast radiography and 9/15 (60%) by hemipenis contrast computed tomography. Outline of the cloacal rim in females was seen in 100% (8/8) by contrast radiography and 100% (8/8) by contrast computed tomography. Our findings suggest that contrast radiography and computed tomography are not reliable methods for sex determination in Gila monsters, and that coelomic ultrasound is currently the most reliable, noninvasive technique for this species.