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Quakers are sexually dimorphic, meaning that you cannot tell which sex they are by their appearance. They must be surgically sexed or DNA sexed.

Surgical sexing consists of putting the bird under anesthesia while an experienced avian vet inserts an endoscopy through a slit cut in the bird's side. The endoscope is then used to visualize the sexual and other organs.

DNA testing can be done by mail; it requires only a few drops of blood to be placed on a microscopic slide; the blood can be obtained by clipping one of the toenails. While this is a safer method of sexing, it doesn't give you the opportunity of knowing if your bird has internal problems such as liver damage, etc.

For more information on DNA and surgical sexing, read this article by Dr. Danny Levinsox DVM.