There is a new book out I haven't seen mentioned here called Field Guide to North American Flycatchers: Empidonax and Pewees
by Cin-Ty Lee and Andrew Birch.
It has detailed descriptions and illustrations of the large number of features that vary, and tells you which flycatchers have each type of feature, as well as other ID tools including overall impression. Features include: crown shape, bill length, bill color, tail length, tail width, forehead angle, wingbar contrast, and more. It suddenly becomes clear flycatchers are challenging to ID because several flycatchers have variable features.
For a sneak peek, check out the "look inside" feature on Amazon. It's all in there.
Weld County, Colorado