Hmm, well, for SOME breeds its easy. For others, not so much. From Backyarchickens.com:

6. When and how can I tell if a chicken is a rooster?

It is VERY difficult to tell male from female at an early age.
Depending on the breed, cockerels (males) develop larger combs and wattles earlier than pullets (females) and will start trying to crow at 6 to 10 weeks (we had one start to crow at 16 weeks - it's not a precise science, sorry!) Cockerels tend to bigger and/or taller than the pullets. You may also observe more aggressive "rooster behaviors", like play fighting, hassling, mounting and such.

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So, yeah, to make it worse, I had none to compare with either- ALL of mine were male. My first clue was actually the aggressive behaviour, not the physical traits.

But, I am not worried either way. The chickens that are roosters will just have to be dinner, is all.

As a side note, BackYardChickens.com is a strange social experiment for me when I see it. It's very burgoise; those people spend a lot of money, love, food, and time on their chickens. It's totally the oposite of here: they're food. They live in boxes made from trash. They run around the countryside, eating whatever they can find. But despite that, i think that the chickens here are every bit as useful and good. I guess there's a chicken for everyone, if they want to take the time to find it.