In most cases, I think you are going to have to wait and see. With some breeds, you can tell the sex by looking at their spread out wings but that only works for the first couple days and it does not work with all breeds. There is also the vent method which involves looking and maybe feeling just inside the vent to see if there is a "nub". With that said though, it sounds a lot easier than it is. That's why hatcheries have professional chicken sexers!