With Liverpool currently languishing in third place after two games, Barnes believes that the Merseysiders will need to be at their best if they want to upset Los Blancos.
"It couldn't be a bigger test, and this is the way it should be," he is quoted as saying by the club's official website.
"I still expect us to go through, regardless of the result, but it will give us a good indication as to how good this team is. And I think that on our day we can be very good."
Liverpool are back in Premier League action on Saturday when they host Hull City.