The most certain method of telling if your ball python is male or female is by having an experienced exotic veterinarian perform a cloacal probe or popping test. However, you can also determine the gender of adult snakes by measuring their length and diameter.
To measure the length of your snake, lay it out flat or place it in a clear tube that is the same diameter as the snake. Males typically do not exceed 3 1/2 feet, whereas females reach between 4 1/2 and 6 feet.
To measure the diameter of your snake, wrap measuring tape around its body at several points to find the circumference, and then divide the largest circumference by pi (3.14) to find the diameter. Males usually reach about 4 inches in diameter, and females reach up to 6 inches.