The empidonax flycatchers are very difficult to tell apart. The safest way to differentiate them is by habitat, range, and voice in the breeding season. Differences due to molt, wear and age make the plumage quite variable. The Dusky flycatcher is larger and paler underneath than the Hammond's Flycatcher. Gray flycatcher is grayer above and rangier. Other empidonax flycatchers have mostly yellow lower mandibles.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.