Ok, that follows with some things we've observed. What you said about the stance, chest etc of a roo. We also notice what we call "hen butt" rounder and fuller than a roo.the gender determination behave your idea has bee passed down through my family. we flip them on their backs and supposedly if it kicks and gets annoyed then porabably a male f it just lies there then it may be a female, this hasn't always been right especially with flightly or skiddish birds or breeds I also add that with how they stand as they age, taller with chest out usually a male and kinda slouched is usually female, hope it works as well for you as its worked for me!
Thought we were pretty accurate and then one threw us a curve...thought for sure this bird was a pullet at four months, even posted a pic and others thought it was a pullet too, then seemingly over night blossomed into this handsome roo. Yet my leghorns, we knew at 4 weeks which were roos, their combs and body structure were unmistakable. Does the flipping behavior follow as they grow or only with a chick?