There are really only 2 definate ways to be sure of their sex. Genetic linked Colour sexing or waiting until you get a cock-a-doodle-doo or an egg. However there are ways to tell with a reasonable level of confidence, but these are not 100% accurate across different breeds.
Lets start with autosexing breeds, there are breeds of chicken which allow you to identify the sex of the birds by the colour of the chick. There are natural autosexing breeds such as the Cream/Crested Legbar and the selective bred hybrids. The selectively bred chickens are generally used at a commercial level and used for meat/egg production, along with a majority of the hybrid layers we see today in our back gardens. The following linked posted by a fellow facebook chickeneer details a lot of the breeds http://www.autosexing.co.uk
Another method commonly seen on the internet is Feather Sexing, this method allows you to determine the sex within the first few days. Males have even length feather and females two distinct lengths, more details can be found here. However this method has mixed results and does NOT work on all breeds of chickens. I have tried this method on over 20 different breeds of chickens with mixed results (this is down to the Dominant/Recessive Gene mentioned in the link)
Comb/Wattle development is always a good detection of cockerel. Generally a males comb will develop and redden significantly faster in a majority of breeds. By 6 weeks of age any birds with a significant amount of comb/wattle growth can almost certainly be a cockerel, but this doesn’t mean the ones with no/little growth an definitely hens. Also note this method is no very effective on crested, top knot or pea comb breeds.
Female Sablepoot Male Sablepoot
You can also tell sex by the shape of the hackle (Neck) feathers, this method is called carding. Place a small piece of white card under the feathers around the neck, males will generally have pointed hackles feathers, where as a female is more rounded. I have included an example picture below; (Left-Hen, Right-Cockerel)
Other indicators such as thicker legs, significant spur development (both males and females have buds where the spur could develop from) are a indication of a cockerel.
I have deliberately left out Vent Sexing, this is another method used at a commercial level. This is a highly skilled job and even the professionals are about 95% accurate.
I hope who ever reads this blog finds it useful, this is based on my experience of hatching different breeds and most of these methods will be accurate on specific breeds, but they are good guidelines to follow. Enjoy Andrew