Another thing you can do is to hold a piece of card or paper underneath their neck feathers. Cockerels tend to have long, thin, stiff-ish neck feathers, whereas hens have shorter, rounded and a bit softer feathers.
I don't think I'd have separated them though. We always have young cockerels running with the rest of the flock, and any fighting is usually short-lived (provided they have space to run away). We only get rid of excess cockerels once it's really obvious what sex they are.