It's only the size and age of them that makes one fly distinguishable from another. When they're all being born at the same time there's absolutely nothing that makes them distinct, and that's your clue that eggs have been laid somewhere in the vicinity.

> It's only the size and age of them that makes one fly distinguishable from another.

Because of their rarity, I find that the ones that wear tiny top hats are easily distinguishable too.