I think she would hesitantly call her "Furina...?" as if she were testing her to see if she would respond to that name, feeling that something was off, then also go "You're not Furina. Who are you?" once Focalors started laughing in her face. Maybe Focalors wouldn't even laugh, she'd humor Clorinde by answering as Furina, but that would be Clorinde's clue that it isn't Furina.