Three identical men came out of the water, and Sukanya knew which one was hers

In the Mahabharata, Princess Sukanya was wandering in the forest when she noticed something glittering inside an anthill. Out of curiosity she pricked it with thorns. It was the eyes of the sage Chyavana, who had sat motionless in his austerities for so long that an anthill had grown over him. He agreed to forgive her father on one condition: Sukanya would become his wife.

One day the Ashvins saw her after she had bathed and urged her to leave her decrepit husband for one of them. She refused. So they offered to make Chyavana young again, on the condition that she would then choose among the three of them. Sukanya went to her husband, and he said, "Do thou so." All three went down into a pool and came out equally beautiful. Sukanya weighed it up, knew her own, and chose him.

In the Shatapatha Brahmana the Ashvins win their soma just the same, but there is no test. Chyavana goes into the water alone, and he is the one who tells Sukanya what to say to the gods. The Mahabharata hands the choice to her. How she told him apart from two gods who looked exactly like him, the text never explains.