Well it really depends on his behavior, At around 4-6 months of age the cockerels will start to get hormonal and breed with your pullets quite often, Which may seem like bad behavior, But I dont get rid of any cockerels until they're at least 8 months old, They'll be more mature by then and...
The first thing I always look at is comb size and color. If it's bigger than your other bird's comb and more pink/red then it's most likely a cockerel, Sometimes it's harder to sex certain chickens by their combs, Rose comb breeds tend to have larger/brighter combs than single comb breeds...