There really is no way to know at that young age. The only way you could know would be if you had an autosexing breed, like crested cream legbars, or if you had red or black sex links. You could also breed for feather sexing, or you could get chicks that were vent sexed. There are also some breeds that have a correlation between certain color characteristics and sex, i.e. female welsummer chicks tend to have thicker 'eye liner' around their eyes than males, but this is not definite. Outside of these things, there is no way to tell at a week old, they are just too young.