i was told with a stanley knife cut a very thin slice off the ooth and if its black it's infertile and if there is a small oose coming out it's fertile. DO NOT cut it clean off though, cut it so it is still attached so you can push it back down like a flap or door.
i did this to one of my chinese ooths and actually cut a nymph out vertualy fully formed, within a week of doing so it hatched : victory:
good luck.