My sexing chicks usually turns out wrong. One method I have started using is the dangleing a ring on a string. I know most are going to say hogwash, but I have been getting better than adverage results. Just take your wedding band and tie it to a string, hang it above the chick in question, If the ring swings back and forth, its a roo, swings in circles, its a pullet. We did this with eggs before placing in the incubator, we hatched 15 of 18 eggs and had 14 of 15 chicks turned out be pullets. My uncle claims he can tell the age of a man or woman using this method as well. One year for every circle or swing the ring makes.
Vent sexing is as much an art as it is science. There is a book available that goes into detail, including drawings, that shows you the method to use and what to look for to determine the sex of the bird. It does take practice, and with my old eyes, a magniflying glass.
Heres the book http://www.cutlersupply.com/cart/index.php?main_page=product_info&cPath=1_17&products_id=65