It is generally accepted that the black band across the ends of the tail feathers in a cock bird will be an unbroken, solid band while the two center feathers in the hen will be mottled and the band will be interrupted by those two feathers.

Actually, it is not too hard to do once the bird is in hand. As Dean stated, the unbroken tail band works most of the time. Also the length of the tail feathers-- if they are over 5 7/8" it is a cock bird--under 5 1/2 it is a hen--inbetween, well then look at the other characteristics. A cock generally also has an orange patch along the upper eyelid area.
The best way, is when cleaning the bird, look at the anatomy-- a cock bird will have 2 small bean shaped testes along its lower back, while a hen will have a light colored foamy looking area-the ovaries. I had the pleasure of hunting for years with a grouse biologist, and every grouse taken was a learning experiance.
As far as telling them apart on the wing, even this eye doctor cant prescribe glasses good enough to do that.

Well ,There you go Harold ,I was figuring just hanging out with you should improve my eye sight and my shooting abilities :rotf: !!!