Vent sexing is pretty hard and takes practice. But what you do is you squeeze the chicks stomach very lightly so wol't hurt the chick but hard enough to make him or her poo. So after she or he poo's then in its vent there should be either a little pimple or nothing if there's a pimple its a...
With ameraucana's its hard to tell but looks like a hen cause of her/his tail feathers. But you might have to listen to the other people about the comb. May you have to wait until she lays an egg or not. At 7 months she would start laying