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From Belshaw (1993):

Key to the genus Dufouria

Distinguish sexes using width of vertex: equal to three-quarters (females) or not more than one-quarter the width of an eye (males). No single character completely reliable (especially in females).

1. Tibia of the middle leg with only 2 bristles on its antero-dorsal surface, tibia of the hind leg with only 2 long bristles on its dorsal surface at the junction with the tarsus (the other 1 or 2 bristles are not more than half their length), male vertex narrower than the distance between the outer edges of the 2 posterior ocelli. [4-5mm in length, male wing with neither of the 2 costal spines longer than crossvein r m] ... nigrita (Fall.)

- Tibia of the middle leg with at least 3 bristles on its antero-dorsal surface, tibia of the hind leg with at least 3 long bristles on its dorsal surface at the junction with the tarsus, male vertex at least as wide as the distance between the outer edges of the 2 posterior ocelli. [5-6mm in length, male wing with costal spines often longer than crossvein r m] ... chalybeata (Meig.)

The 2 species are sometimes quite difficult to distinguish but chalybeata is the commonest in my experience.