One golden rule of thumb that I’ve learned from advanced and expert hawkwatchers is: shape trumps plumage! Plumage details are easily lost at a distance and/or obscured by a variety of lighting conditions. It helps to understand shape and flight style; both of which require an in-field “feel” from many hours of study. Here’s a neat one: a Northern Goshawk “dressed” as a Cooper’s:
Photo: Note the overall massive build consisting of: thick wings with an extensive bulge to the secondaries (sinuous trailing edge); very broad chest and body (dwarfing the width of the head here); long and broad tail.