I have always remembered this scene in the book vividly for some reason. The old lady tosses a ball of yarn to Huck, and it is Huck's response that reveals he is not a girl. Huck is accustomed to wearing pants, not a skirt, so he clamps his legs together to catch the yarn. Girls, who were in those days accustomed to wearing skirts and dresses, were likely to spread their legs apart so their dress or skirt would catch the ball of yarn. The old lady is fully aware of this difference, so she knows now that Huck is a male.
When I was growing up, girls did wear skirts and dresses most of the time, so this kind of "test" would have been accurate through most of my childhood. But in today's world, few females, at least in Western cultures, wear skirts or dresses most of the time, so it is difficult to know what a typical female reaction might be in this situation.