[Bloody Sam Roberts]

If she wasn’t Arya how else would the hound, her and Beric Dondarrion would have know that Dondarrion was on her list?

Hound: "Is he on your list?"
Arya: "At one time, yes."

Sandor 'The Hound' would have no way of knowing if he was or wasn't on the list. 'Arya' could have simply been bullshitting him. Same for Dondarrion.

[marajade]

They spent a lot of time together. Who else, besides the hound, even knew Arya had a list.