I can guarantee that both the string trick and holding the chicks on their backs will work about half the time. There are a million of them. Hold the chick up by its beak, hold the chick's beak closed, hold something in front of the chick, hang the chick by its feet... I have heard them all and I have seen many, many, many reports of each and every one of them failing to work accurately. They should not be taken as a reliable way of sexing chicks, no matter what an 'expert' at it might say. Hatching autosexing or sexlinked birds is the only way to know absolutely the sex of the chicks at hatch--even vent sexing can't be guaranteed beyond 90% accuracy. Other than that, because the time for both vent sexing and feather sexing has passed, you'll just have to wait it out. The sex of a chick can be determined between 4 and 6 weeks of age in most birds, a little later than that in some because of their breed or how they develop. The males shouldn't start crowing until long after that, so you can always post their pictures in a couple weeks and see then what you have.