Usually by 4 mos I can see saddle feathers coming in, which look much different than a pullet's cushion. They will be longish (not too long at this age) but have a definite point instead of a nice round feather. At 6 mos, hackle is beginning to come in, again very pointed...the pullet's hackle is not as thin and pointy as the cockerel's hackle. It is much easier to see if you have more than one to compare to each other.
Another thing to think about (if you only had one) is that at 6-8 weeks, the males will still have fluff on their backs instead of real feathers coming in. The females will be much more fully feathered at that age, although if you pick one up and look, you can see that they still have some spots where the feathers aren't quite in yet....I'm guessing that their feathers just cover better when they are younger.
Hope that helps a bit.