How does the loyal nurse Eurycleia recognize Odysseus?

When Odysseus was hunting on Mount Parnassus as a boy, he was injured by a wild boar. He did kill it, but it got a piece of him as well. It was a cut that ran just up from his knee.

Eurycleia nursed Odysseus when he was a boy and she recognised his scar - this is how she knows the beggar is actually Odysseus..