According to John Hollister, Western hognoses can be visually sexed by looking at the vent (cloaca) and body behind it (the tail). Females have short, stubby tails while the males have longer, more slender tails by comparison. Adults are easier to sex than hatchlings.
Due to the small size of North American hognoses, this method of visual sexing is considered a much safer alternative than traditional probing or popping.
Other pictures comparing males to females can be found here at ReptileForums.