Like Norm, I find the field mark of downward tail wagging to be overly emphasized and often confounded by birders. The field mark that I find to be most reliable is the unique bill shape of Gray Flycatcher, being longer and with a shallower depth than other Empids, and having a distinct awl-shaped lower mandible. The lower mandible is always a nice clean orange color with a contrasting dark tip. Combine the bill shape/color with a round head and a bold pale lore, and then maybe a behavioral clue like tail dipping. Using this as guidance, it's clear that all reports of Gray Flycatcher in the Denver Metro area this year are 100% accurate.
Christian Nunes
Boulder, CO
David Tonnessen
Colorado Springs