It depends, if you happen to have a color pairing that results in gender linked color in the chicks you can tell as soon as color is apparent. ***
If you have been breeding a particular line or family of birds for some time you will learn to sex most of them at a younger age.
Otherwise it is hard to be sure unless an egg is laid.
The bowing and cooing and tail dragging displays of cockbirds are a strong indicator of course.
You can try to assess the distance between the pelvic bones- wider in hens narrow in cocks but this may not be as clear until egg laying has started.
If you consult google/youtube you can find many 'sure fire' theories. I remain skeptical although some are interesting.
I just had to wait until they started breeding to figure it out.
*** i just posted about a color sex-linked youngster here
https://www.backyardchickens.com/threads/pigeon-bulletin-board.1174207/page-41#post-20113638