Try comparingit to something you are familiar with. For example, say we've sighted a bird and are trying to determine whether we are looking at a peregrine or a kestrel. The kestrel is a small falcon, related to the peregrine but more common. The female kestrel will be all rusty brown and the male will be rusty brown with blue-grey in the wings and on the head. They have double black stripes on each side of the face.
Peregrines have only one stripe which is fairly wide, on each side.
Immature peregrines will be brown with heavy vertical striping on the chest. Adults are dark to light grey with horizontal barring on the chest. If yes has been answered to any of the above questions, it is most likely not a peregrine. Kestrels bob their tails when perched and hover when hunting. Kestrels and Sharp-shinned Hawks are found at feeders more often than any other species around here.