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 Hammond's flycatcher is typically smaller and darker than the other western United States empidonax flycatchers. It is especially dark on the breast and grayish on the throat. Eastern empids have paler lower mandibles and paler plumage.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.