The most accurate way to sex a ball python is to have a qualified reptile vet or experienced breeder probe it for you. "Popping" is also an accurate method if done correctly. Either of these methods can cause serious injury to the snake if done incorrectly. The size of the spurs and/or shape of the tail is not an accurate way to determine the sex. If you have any other question you might want to check out this link it's really good.