How does Odysseus prove his identity to his servants eumaeus and philoetius?

In order to prove he is who he says he is, Odysseus follows both Philoetius and Eumaeus outside, Once there, he guarantees their loyalty to each other and identifies who he really is via a scar that is located on his foot. Once done, he promises to treat them as if they were Telemachus's brothers if they can promise to fight by his side.