One thing to be careful of, or take note of (especially if you are a bird bander), is how certain patterns may look odd on a bird's tail but are actually the norm. For instance, Harriers often have darker or less marked central tail feathers compared to the rest of the tail. A male Kestrel's outer tail feathers (and often the last several outers) are banded, while the rest of the tail is not. Adult Goshawks have muted bands on the tail but often the central feathers are even more muted, or some have outer tail feathers that differ from the rest.

There are other examples in the raptor world, but these are just a few, check out the pics below to see some examples ("click" to enlarge). I commented previously on the difference in the outer tail feathers of adult Broad-winged Hawk: http://jerryliguori.blogspot.com/2013/07/broad-winged-hawk-tidbit.html

 Juvenile Northern Harrier

 Adult Male American Kestrel

 Juvenile Male American Kestrel

 Adult Northern Goshawk

 Adult Northern Goshawk

Adult male Northern Harrier