I'm quoting the book I have, " In a dimly lit room, place a small light against the tail base of the corn. On males, you will see two dark red lines running from the cloaca toward the tail tip. These represent the blood in the hemipenes. These lines usually extend seven to 14 sub-caudals from the vent. If you see any small, red lines in the female's tail, they will be from the vent, just three to five sob-caudals back. These represent the hemipenial homologs & are essentially functionless in females." It also says thats its only effective on very young snakes & only on pale morphs.