That's like the string/ring test which works for horses & people...at least so far.
You take a piece of string and slide a ring onto it. Then hold it above the pregnant one's belly. If it goes in circles, it's a girl. If it swings back & forth it's a boy. So far it's been 100% accurate with my horses and my own pregnancy. Not sure if it's luck or just being on the right side of the 50% chance each time.