How does Wen-ch'iao guarantee that she will know her son if she meets him again after sending him down the river in the novel, Monkey?

Afraid for her infant son's life, Wen-ch'iao saves him by sending him down the river so her impostor husband won't have him killed. She bites off a piece of one of his toes to insure that she will recognize him if she ever finds him again.