It all depends on breed. Some breeds are very easy to tell that they are roosters. Others, not so much. I had one breed that starting crowing at 4 weeks of age (Egyptian fayoumis). But most don't start to crow until around 4 months of age. And it's hard to catch them in the act of it!
The neck feathers are much more jagged and pointed at the neck area on roosters, and very much rounded at the tips on hens. However, this difference doesn't show until 4 or 5 months also.
The roosters will have longer tail feathers. Larger feet, larger bodies also. They do not necessarily behave any more aggressively at the beginning. In fact, the most aggressive one is almost always the head hen.
Last year, we hatched 15 eggs---10 ended up being roosters! 5 were hens. The year previous we also hatched 15---8 were hens, and 7 were roosters.
I bought one chick from the feed store last year. ONE. It ended up being a rooster. Grrr.....
Having three human boys, it seems to favor my odds of having more roosters than hens.
I am hoping my 9 chicks this year are all girls.
