The problem with mystery chicks is they have a giant assortment that you can't tell at a young age (you would have to be with the breed for awhile and you have to be accustomed to that variety that you've been breeding because certain colors in a breed will look different from each other) I would say to wait a couple months where they start to develop features that are easily recognizable. Also look for physical features that may be different than others. (How many toes? What kind of comb? Does it have feathered shanks and feet? or even a muff, beard or starting point of a crest?)