The Dusky Flycatcher is one of those troublesome empidonax that is almost impossible to distinguish by sight alone. It is easily confused with Hammond's Flycatcher. However, by piecing together typical habitat, behavior, voice and previous sighting information, it can be done fairly reliably. The below bird was photographed in Florida Canyon near the famed Madera Canyon in Southeast AZ. Dusky Flycatchers tend to stay out in the open, lower down and in more scrubby riparian habitat. Hammond's on the other hand tend to hand out higher up in coniferous areas. I ended up seeing a Hammond's on this same trip a couple days later over in Ramsey Canyon which is definitely chock full of conifers and true to form it stayed higher up.
I believe this particular bird was the same bird that some other very good birders (eBird reviewer) had also reported in Florida Canyon on either side of my visit. Note the relatively short primary projection.
Note the relatively rounded appearance of the head. Hammond's has a more peaked looking head.
Here is presumably the same bird taken later in the day when I brought my brother back to look for the close by Black-capped Gnatcatchers.