Hi Ian, I don't know how you do this with cpqs, but if you go to YouTube and
Search for Sexing Chinese Painted Quail or Sexing Button Quail you might find something useful. With my coturnix it was fairly easy to sex the wild colour brown ones by feather colour, but with some of the fancy colours I had to wait until the boys started producing foam balls when I squeezed their backsides (sorry!) I or when they crowed and started raping the girls ( sorry again - coturnix have no manners.) I'm not sure if cpq males produce foam balls at puberty, but this is reliable of course, if they do. Of course once the girls began to lay eggs that was useful, except working out who had laid was a bit tricky!
So I shall be interested in anything you find out about sexing cpqs.