Chromosomes. Like in humans.
Except that in humans, it is males who carry 2 different chromosomes (XY) while females carry two copies of the same one(XX), so that it is whichever one is in a particular sperm cell that determines the sex of a particular offspring.
Whereas in chickens, it is the female who carries the 2 different chromosomes (ZW) while the male carries two copies of the same (ZZ) so it is effectively the HEN whose reproductive cell (ovum) determines each chick's sex.
You may be thinking of certain reptiles, such as crocodiles and alligators, where sex of the offspring is determined by what temperature an egg is incubated at. T'aint that way in chickens.
Pat