I was the R, and was looking at the serving team. The U was watching the receiving team. And since we were using volunteers for line judges and scorekeeping, some games the best we could get was somebody to flip the numbers on the flippy-scoreboard-thing. So no actual scorekeeper that game.
My U did not use a lineup card or wheel. He only had the submitted lineup to work from. Still...how difficult can it be? The team was serving, and they didn't sub out their setters. So start with the server, back up two to the CF and see if the FR setter is supposed to be in that position. If not, it's her. If so, it's the BR setter that's out. And while checking the back row would be slightly more confusing since the libero was in the game, the libero's the only one she would be overlapped with.
As a side note, I was going to try and work some of these games without the wheel, just to see how I did. But the first game I was going to try, we didn't have a real scorekeeper. Perhaps it was caution, perhaps it was cowardice, but I didn't want to be in a position where nobody knew for sure what the current rotations were supposed to be. So I never did try without it.