It's about the length of the flight feathers and it has to be like the first 3-5 days I think. It doesn't work on all breeds. Sorry that's all I know. Can't remember if the long feathers are male or female. Sorry.
i think it has to be done in the first 3 days and if the feathers sit level in the row its a boy if there are two clear layers of different length its a girl but it apparently doesn't work on all breeds (no idea which either sorry) and having tried this method to sex my recent hatch i would say its not easy even using Google images to compare!!