Wing sexing is judged by the length of feathers already growing in. Pullets mature faster and have crooked feathers coming in, whereas cockerels mature slower and have all feathers come in evenly. However, you have to be a professional wingsexer to accurately tell. Even then, the accuracy isn't great.
You have to do with in the first 3 days of hatching and you are looking at the primaries and coverts of the wings. How ever having had japs and breeding mottled I have found that for my particular line they are not wing sexable. Wing sexing is can only be done on chicks that have the fast feathering gene in their lines which means only a handful can actually be wing sexed.