The average person cannot sex a chick until it grows out or they breed for things like feather sexing. Most people don't. Hatcheries sex chicks by looking in the vent to determine gender. Even those are not 100% accurate. Feather sexing by hatcheries is possible because they breed fast feathering hens to a slow feathering roo or vice versa. If you don't breed this way, the feather sexing does not work.
I can usually tell the sex on most breed by 4-5 weeks old, altho some breeds take much longer.
There are a few breeds that you can sex by collor or patterns Sex links being one of them because thats how they were bred. Another one is speckledhens fave. Barred rocks. it works 90% of the time she says. I am going to put her to the test once I get some chicks