It's basically there from hatch, in the chick down and then the feathers.
Cuckoo is white barring on black, just not as neat and tidy as the ones called "Barred."
So the chick would hatch out the same color as Barred Rock or Dominique chicks (black with some yellow underneath, and often a yellow spot on the top of the head. A yellow headspot should always mean barring, but a chick who does not have a noticeable headspot might have barring anyway.)
The white barring should be present in the feathers from the very beginning. Sometimes it is more obvious than others, but if the chick has quite a few feathers and they are all smoothly black, it is not cuckoo. Or if the feathers are entirely white, it is not cuckoo either.
If you have chicks that you are curious about, you can post photos and someone here can probably tell if they are cuckoo or not.