You'll know the difference by the way Scuba reacts, she'll either be willing to let the lil one come close or she'll start to pick on it. when first meeting maybe have a bit of distraction like some dried meal worms for Scuba, baby may not be interested in them right off but may entice scuba to focus more on eating the worms than the duckling. Hopefully Scuba will take right too the baby. I had single ducklings hatch 4 weeks apart a couple years ago and the 4 week old took right to the new duckling they have been inseparable ever since, Youngest duck was with a mama duck so I didn't have to worry about older one getting to close until the younger one was stronger.Ok, we will be very cautious!! Will it be obvious if Scuba isn't accepting the little one? Any signs to look for?