One way I use to sex offspring is by the colors, since I'm a color genetics nerd

There are certain sex-linked matings that will give you a color that is always hens, and another that is always cocks.
Also, if you have an ash-red bird with black/blue or brown flecking in the tail or wings, then it's a male. The flecks are an indicator of that red bird carrying another color (blue or brown). Hens can only carry one color (the color you see), so therefore they won't have any flecking. SAYING THAT, there can be cocks WITHOUT flecking. It just means it's a pure ash-red, not carrying any other colors.
Other than those color tricks, it can be hard to tell until someone lays an egg. They like to trick us with their behavior. But one thing that has always worked pretty good for me is feeling the pelvic bones. They come down to a "V" at the vent. Hens require a space between those bones for the egg to pass. Young hens may have tight vents until they lay for the first time, but typically the bones will at least be pretty flexible. Cockbirds on the other hand, will have close, tight spaces, sometimes with the bones touching. The younger the bird, the harder to read. The males may feel like hens and the hens may feel like males, until someone's either tightens or loosens.
Still, with a bird about 3 or 4+ months old, you should be able to get a pretty good guess.