Probably not that obvious from a distance but there's no great secret about the time that's to be added on. The TV floor manager will ask the 4th official at the end of each half so the commentators and graphics people know in advance, and s/he is right by the benches. Also, close up, you can see the amount of time being added on a little LCD screen on the back of the board they hold up, so the official can see it's right before showing it to the crowd.