Usually it's pretty difficult to tell for sure at that age whether its a chick or rooster but keep an eye out for a few things;
Rooster chicks (cockerels) will start to develop their comb quicker and at around 5-6 weeks( at least for me) the comb of a rooster will be pink where as a hens will be smaller and a yellow color.
Hens usually grow tail feathers quicker than roosters
Roosters tend to be more active and 'spunky' my bantam rooster chicks are 5.5 weeks old and they are running around, jumping all over everything, pecking at the others and essentially bouncing off the walls. My pullet chicks on the other hand are a lot calmer and sit around more.
Hope that helps a bit