The safest way to differentiate them is by habitat, range, and voice in the breeding season. Differences in plumage due to molt, wear and age make the plumage quite variable. Willow and Alder appear longer-tailed, have less obvious eye rings, are browner above and whiter below.
Least Flycatchers are grayer above and whiter below.
Acadian Flycatchers have larger bills, longer tails, less yellow on the throat and are less active. Empidonax flycatchers from the western United States typically have darker lower mandibles and different ranges. Cordilleran and Pacific Slope Flycatchers are somewhat larger with more oval eye rings. Pewees are larger with whiter underparts and less obvious eye rings. Birds of North America.