Hello Richard,
Yes, that is a good question! :-))
If a specimen is available, it is usually very easy to identify a specimen to genus. With a photo, one is restricted to the information that is available on this photo!
In this case, the difference between Brachypalpus and Pterallastes lays in the wing venation that is visible!
In the nearctic identification key to syrphid genera they separate 2 groups at a certain point, of which one group has a certain feature in the wing venation(vein R4+5 straight or nearly so, not strongly dipped into cell r4+5). The other group does not have this feature.
So, the genus Brachypalpus falls in the group which DOES NOT have this strongly dipping in the wing cell, and pterallastes DOES have this feature.
With some trouble (because of the folded wings) and experience it is possible to see the difference, but nobody says it's easy! :-))
Greetings,
Gerard Pennards