Only chicks that have been breed to be wing sexed can be sexed by the wing feathers as day olds.
Never try and vent sex. It takes years to learn to do it correctly and if you want to learn, expect to lose quite a few chicks before you get the hang of it. Even once you get good at it there is at best a 90% chance you'll be correct. There are books on the subject if you want to learn more. Basicly there are, in my experiance with vent sexing, three types of vents. About a third are obviously male, about a third are obviously female and about a third you just have to use your experiance and make your best guess.
It's a bit hard to explain, but there are books on sexing that shows the differences.
BTW, I never even vent sex my own birds, even though I can and have in years past.