Both versions of Gladiator are labed "Sapphire Series." The remastered Blu-ray wasn't a formal re-release; they pretty much just put a new disc in the old package. The new one does have a yellow (instead of white) UPC to distinguish them, but I doubt there's any easy way of telling which version you'll get if you order online. Based on recent user reviews, it sounds like Amazon's still shipping the old version.
Since Paramount has an exchange program in place, it shouldn't matter too much if you don't mind waiting. If I really wanted the remastered disc right away, though, I'd try looking for the "yellow UPC" version in stores.