In The Odyssey, there is a scene that takes place near the end of the story. Odysseus returns home after many years of wandering. He is in disguise as an old man. At first nobody recognizes him, not even his wife and child. One night before bed, Odysseus' aged nurse bathes him. At first, she thinks he is just a stranger; but while bathing him, she recognizes a scar on his leg. She remembers the scar from his infancy. She did not recognize him until she saw his scar.
Thomas had a similar experience. When he saw the scars, he new the resurrected One was the crucified One. He knew it was Jesus.