Rosemary Hitchens asked for opinions on whether the hawk she'd seen at Marymoor Park on Sunday was a Cooper's Hawk or a Sharp-shinned Hawk.
When I looked at her photos, my guess was Sharp-shinned based on size and the squared-off tail. However, I had very little confidence in my guess, particularly because size is hard to guess when there is no comparison available to provide relative size. My husband, Randy Robinson, guessed Cooper's, citing no supercilium, puffy white undertail coverts, and length of tail. But he also thought it was hard to tell.
I ran these photos (using a cropped screenshot) through Cornell's Merlin Photo ID (a smartphone app), and it identified both as Cooper's Hawk.
Also, here's an Audubon article about distinguishing between Cooper's and Sharp-shinned: