I'm kinda convinced that vent sexing is at least as accurate as sitting around for a few hours waiting for someone to yell. I just sexed out a couple dozen boys for freezer camp before I sat down here.
It's true that I've yet to see a female crow; but I've also yet to see a female pour a giant glob of foam out of her bottom when you tugged on her tail.
Just saying
Depending on how old your quail are by now, it should be ridiculously easy to flip them over, bend their tail back, and see if you open up a red, empty cavity, or if you see foam or bubbles come out. The males that you have isolated so far would be great to begin with.
It's easy. I swear. Grab the bird and hold it firmly in your hands. grab the tail with your thumb and push back, which will open up the vent. Here is the sexing key:
Poop: Female
Nothing: Female
Bizzare glob of shaving cream: Male
The more mature the male, the thicker the shaving cream. When they are young it may look more like bubbles.
If you don't see it on the male that is crowing then they may be too young (if only by a few days). If you do see it on the male then check all of your others and band the boys. You can always check the females again in a week to see if you had a late bloomer in there.
Once done, wash your hands, kick back, and enjoy the rest of your beer.