Supreme Court justices questioned the underpinnings of a massive class-action lawsuit against Wal-Mart on Tuesday and whether female employees could show that a common, discriminatory policy governed the company’s pay and promotion decisions...Even justices who seemed sympathetic to letting the largest gender discrimination lawsuit in history proceed to trial had questions about how it might go forward...[Some] justices said they had trouble understanding how, in the plaintiffs’ view, Wal-Mart carried out its policy of discrimination...[Some of the justices'] questions echoed Wal-Mart’s argument that lower courts should not have certified a class-action suit against the company...The court’s liberals...said that simply adopting a policy of nondiscrimination was not enough to insulate Wal-Mart from liability...But even Sotomayor and Ginsburg expressed concern about the suit.