I recently read a great book, "Why Did the Chicken Cross the World" by Andrew Lawler. He mentions chicken sexing in the book via the process already mentioned here - the bump inside the vent indicates a male. But apparently it is as much art as science, said something in the book about if you try too hard you won't get it right and that only certain people have a gift for consistently being able to do it correctly.
See my chicken blog at: http://polloplayer.wordpress.com/
My little flock includes Pippa, a Belgian Mille Fleur d'Uccle, Ginger, an EE and three babies added January 2016 - Bella the BO, Ava the Australorp and Nugget, a RIR.