While genus ID for most EPT taxa requires microscope work, there are a number of families where the genus can be nailed down by volunteers that monitor streams, certainly if they're using a loupe. That is the case for the Heptageniidae -- flatheaded mayflies -- so I've worked up a cheat sheet for those who might want give this a try. Just be advised, that this only applies to the flatheaded genera I've seen. The two I've not yet encountered are Macdunnoa and Stenonema. Both are type 1.
1. with thin, single filament gills on segment 7
d. maxillary palps visible on sides of head -- Cinygmula
a. Heptagenia. You can see those "frilly filaments" behind each of the gills.
b. Leucrocuta. There are strands of filaments (nothing like the fibrilliform on the Heptagenia nymphs) behind the gills on segments 1-6. They're absent on 7.
c. Nixe. This genus is uncommon. I've only seen it in the Pacific Northwest, but according to Beaty they are found here on occasion.
Nixe cannot be distinguished from Leucrocuta out in the field. It takes a microscope to see the "intercalary setae" on the cerci (tails), present on Nixe, absent on Leucrocuta.
Still, it's a safe bet that the "fatheaded" flatheads you find are genus Leucrocuta.
I can tell this one at sight -- others might need a loupe. Look for two bumps on the sides of the head, the maxillary palps.
Not so tough after all, right? And it might add to your enjoyment of picking bugs off the net! Which genus are you most likely to see? Maccaffertium, no doubt about it. These nymphs are in our streams year round, the other genera are seasonal, generally speaking in spring and summer.