Just thought I'd throw in a couple of flight photos, giving a few clues to accipiter's appearances in flight. The almost straight line of the leading edge of the wing allows the large head to be easily seen on Cooper's as it extends forward of that line.

Sharp-shinned with a full crop will fly with their small head extended but it is still a relatively small head. The Cooper's tail, in most instances, appears long and rounded but the loss of central tail feathers can give it a more squared appearance. The white on the tip of the tail is usually wider and more obvious than on Sharpies. Good points but should be used with caution to build a case on multiple points. The best bet is to study every bird as long as it is in view.