The comb and wattles on cockerel chicks will turn dark pink/ reddish much much sooner than on pullets, except on breeds with black or very dark combs.
There are noticeable differences between the sexes in some specific breeds, most commonly the auto-sexing breeds and sex-link hybrids.
The auto-sexing breeds include the barred and cuckoo breeds. Those females feather in darker than the males. At hatch the females also have a smaller white spot on their head and the males have a larger one.
Sex linked birds are created when a genetically gold/red male is bred to a genetically silver female, then the female chicks have gold down and the male chicks are yellow.
Some breeds can be feather sexed. In that case, there is a difference in the wing feathers during the first week only.
A more subtle difference can sometimes be seen in their behavior. When startled, the pullets are more likely to duck for cover and the cockerels stand up tall and look to see where the threat is coming from. But this is by no means a sure fire test of sex. LOL