Well..........you can not tell the gender of a chicken by looking at the egg. There is absolutely now way to guarantee a total hen hatch rate. About half of the chicks that you hatch in a batch will be roosters. It's the sad sad truth......
Therefore saying your chances of hatching hens will be fifty-fifty.
Now, buying all female chicks is a different story. Once chicks hatch in the hatchery, they are sent to the sexers. The sexers are highly trained people that check the gender of the chick. They can tell by the pattern in length of the feather tips if it's a boy or a girl. They also sex them by literally squeezing the **** out of them, once the poop comes out, they quickly check the vent area and determine it's sex before it closes back up again. These methods originated in Japan, and only some of the old time Japanese people know how to do this properly. It takes lots of skill and highly trained eyes to look for the right things. Most of the hatcheries that use the squeezing method hire these Japanese people, since they are pretty much the only ones who know how to do it right. It takes years of experience, and they can still only guarantee 90%accuracy . Heres a great video that kinda helps explain the procces http://www.youtube.com/watch?v=1tmEO9xRqvo
If the breed is a sex link, any person could tell by just the color of the chick weather it's male or female.but usually thats not the case. Once the chicks are sexed, they are shipped to feed stores where you can buy them, or you can order directly from the hatchery. Hope this helps