And let it come to pass, that the damsel to whom I shall say, Let down thy pitcher, I pray thee, that I may drink; and she shall say, Drink, and I will give thy camels drink also: let the same be she that thou hast appointed for thy servant Isaac; and thereby shall I know that thou hast shewed kindness unto my master.
So, a girl will be told to lower her pitcher so I can drink. If she agrees and offers water for my camels too, then she is the one meant for Isaac, proving you are kind to my master.