The best way to tell dark-morph from light-morph Rough-legged Hawks from above is that dark birds have all-dark tails (with or without narrow, whitish bands), and light birds have a white base to the tail. Simply...only light birds have the white tail base on top. However, a few light-morph birds have very limited white to the topside of the tail. Note the tail pattern of the light-morph bird on the bottom left compared to typical light-morph tails (middle and right).

I added some dark-morph photos above the light birds per request of Robert. Note the dark topside to the tail on the middle (adult female) and right (juvenile), and the narrow but bold white tail bands on the adult male (left).

Here is a gorgeous photo of a dark juvenile (top) that was taken by my friend Vic Berardi....check out the paler smudgy bands on the top of the tail shown by some dark juvs, and the pale primary "panels" or "windows".