Behind the eyes- in a region known as the occiput- there is ridge that extends from eye to eye. Neoperla share this feature with the genera Agnetina and Paragnetina. In contrast to those, however, they have only two ocelli instead of three. Occasionally you may see specimens which appear to have only two ocelli, but be careful! Sometimes shining a light source sideways can reveal a third ocellus that has yet to appear, making it not Neoperla and probably an immature specimen of some other genera.
Here is a close-up of the head showing both of those features.