How does the old servant know it is Odysseus when she washes his feet?

When Odysseus returns to Ithaca in the disguise of a beggar towards the end of Nolan's The Odyssey (2026), how does the old servant woman with white hair know that it is him just by washing his feet?

She recognizes him by a distinctive scar on his leg. Here is the relevant passage from Book 19 of Homer's Odyssey, translated by Samuel Butler (from Project Gutenberg; emphasis mine):

Then the old woman took the cauldron in which she was going to wash his feet, and poured plenty of cold water into it, adding hot till the bath was warm enough. Ulysses sat by the fire, but ere long he turned away from the light, for it occurred to him that when the old woman had hold of his leg she would recognise a certain scar which it bore, whereon the whole truth would come out. And indeed as soon as she began washing her master, she at once knew the scar as one that had been given him by a wild boar when he was hunting on Mt. Parnassus with his excellent grandfather Autolycus—who was the most accomplished thief and perjurer in the whole world—and with the sons of Autolycus.