They watch them on film on a monitor - not on the actual dance floor. That way, they can openly discuss what to say and score with the producers. Otherwise, it could hardly be kept a secret and their chatter would be a distraction to the dancers, director, etc. They also have access to all the rehearsal video but the judges wouldn't really need all that - just the producers, as they decide how to "guide" the show. I think they know who'll win before the show even begins (and also where everyone will place), or at least what their plans are, but as the show progresses, they make adjustments, depending on viewer votes, to some extent, but also how things unfold, injuries, personalitybehavior issues, etc. They don't leave anything to chance. No reality show does. Too much money riding on it.