The first sign to look for is combs that are more red/pink than others. Some show that at 3-4 weeks old. The cockerels' combs often grow bigger & look more red than their pullet sisters, and their legs look thicker. Their wattles start showing sooner, they look like adolescent boys growing their first beards.
At around 5-7 weeks it should start to be more obvious by their appearance. At around 9-12 weeks some start to crow, screechy wimpy little attempts. Then their tail feathers grow longer, and the pointy hackle & saddle feathers begin to sprout.
The time frame is an estimate. Some breeds/individuals show these signs of maturity sooner or later. Some guys look & act manly almost from hatch, others will keep you guessing for months.
By the way, chest bumping, hackle raising, hopping & spurring is done by both boy & girl chicks.
Cockerels will be trying to mate weeks earlier than the pullets wish. They'll be trying to pin the girls down while they shriek & fuss. But they won't hurt them, you needn't separate them. A good ratio is 1:10-12, that should keep the boys busy & out of trouble with each other. Individual results may vary. Some roos can live very peacefully alongside others, while other roos will beat the giblets out of any other roo in the yard.