If you want to find out if she is laying or who is, use food coloring around the outside of the vent then when the egg is laid the egg will have the food coloring on it. Do different color for each duck.
It could be she is laying internally but they usually don't last long when that is going on...
Does her tummy area feel squishy and different from her sisters? They can get fluid build up inside called ascites. Probably her poop has discolored her feathers.