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There are several different ways to sex chicks. The basic ones based on genetics are the red sex links, black sex links, and feather sexing. These will work real well if the parents are set up right and the person sexing the chicks know what they are doing. Some breeds, like the Barred Rock, can sort of be sexed based on differences in the chicks when they hatch. This is less certain, but you can get some real good clues if they are purebred barred rocks, for example.
Then you have the vent sexing. This is where someone very well trained looks inside the vent and determines sex. This method is not totally foolproof and depends a lot on how good the person that does the sexing is. This is the one they guarantee 90% accuracy, which to me means they are probably a little better than 90%. This method is only used on full sized breeds. They won't even try it on bantams. It is not that easy.
Your best bet to get pullets is to buy the sex links if you buy baby chicks. From a feed store it gets a little more difficult because of all the possibilities for mix-ups others have mentioned. It is usually not the hatchery that has the problems. They do chickens as a main business. Many feed stores do not have people that well trained in chickens. As in many other things, if you don't know what you are doing, you can make a mistake.
Another method is to get POL (Point Of Lay) pullets. These are chickens about 16 week sold. By then, you should be able to tell which are pullets and which are roosters. The possible downside to this is that it is easier to bring a disease into your flock. The hatchery chicks are real safe relative to disease, but older chickens are a little more likely to carry diseases. People add older chickens to their flocks all the time and usually don't have a problem. But there are times that flocks are wiped out. If you go this route, check out how to quarantine.
Hope this helps a bit. Good luck and once again,
