This depends a lot on the flavors of birds you have in your area. The high contrast you often get with birds soaring often makes color or markings difficult or impossible to tell, and size can be notoriously difficult to estimate if birds are soaring high, so shape and behavior become much more useful criteria. For behavior/movement, I’m looking at circling vs. direct flight, flapping style (deep or shallow wingbeats, sporadic or continuous), and stooping, hawking, or other hunting behaviors. For shape, consider ratios of tail length to wing length, as well as general wing shape (broad, pointed, etc). Hope that gives some pointers, a lot of times it just takes the hours in the field to get accustomed to how all of those factors come together for identifying each species. For instance, Turkey Vultures tend to hold their wings slightly upward, and have a bit of a “wobble” sometimes when soaring where they appear a bit unstable, that can often give a easy feel fro what species it is.