With African and Chinese you can sex relatively accurately by the size of the knob, but because yours are crosses it may not be as accurate. The only way to know for sure is to vent sex. Which is not as hard or as gross as it may seem at first.
Grab a goose, take a seat. Turn the goose upside down with it's head extending out behind you, under your dominate arm. Rest the bird's body on your knees, with its feet pointing towards the sky, its tail pointing away from you. Part the down around the vent, holding the skin around it taut and with your other hand massage the vent open. If you are able to expose a long, white, spiny organ -- that's the penis -- it's a male. If you do not find a penis, it's a female (or you did it wrong). Wash, rinse, repeat. With practice you will learn what the internal female genitalia tend to look like as well, which will help your efficiency and speed.
As a side note some people sex with the goose tail facing them, you can try both ways and see what works best for you. I feel like I can get a better hold on the goose with its tail facing away from me.