The most useful character for identification from other dipteran groups is antennal shape/number. The third antennal segment is either annulated (but not many-segmented as in Nematocera) or stylate/aristate. Groups that have aristate antennae can be confused with Cyclorrhapha, but can be separated by the lack of a frontal suture. One exception to this is Rachicerus, which has antennae similar to Nematocera.
"In some Brachycera the third antennal segment is subdivided, but the divisions are not as distinct as those between the three basic segments, and such a segment is said to be 'annulated'. This annulation is sometimes difficult to see unless the antennae is properly illuminated. In a few cases it can be difficult to decide whether such an antenna is three-segmented or many-segmented. The third antennal segment in many Brachycera bears an elongate process: a styles or an arista. A style is usually terminal and fairly rigid, whereas an arista is usually dorsal and is bristlelike. Both styles and arista can appear segmented, although the segments (particularly in an arista) are often difficult to see."(1)
Some more technical characters:
1. Reduced antenna size (with 8 or fewer flagellomeres).
2. Maxillary palpi having 2 or fewer segments.
3. Posterior of the larval head capsule extend into the prothorax
4. Two distinct parts which comprise of the larval mandible (lower jaw).
5. Epandrium and hypandrium of the genitalia are separated in males.
6. No premandible on the lower surface of the labrum (the roof of the mouth).
7. A distinct configuration of the CuA2 and A1 wing veins.
8. Lack the circular pupal aperture that characterizes the Cyclorrhapha.
Many of these flies are large and attractive, and are of considerable interest to naturalists. Some are irritating and painful biting flies of humans, pets and livestock (Tabanidae, Athericidae and Rhagionidae). Larger Asilidae are known to deliver a painful bite if handled carelessly. Asilidae, Bombyliidae, and Dolichopodidae are very species-rich families, each with close to a thousand or more spp. in our area.
Shin, S., Bayless, K. M., Winterton, S. L., Dikow, T., Lessard, B. D., Yeates, D. K., Wiegmann, B. M., and Trautwein, M. D. 2018. Taxon sampling to address an ancient rapid radiation: a supermatrix phylogeny of early brachyceran flies (Diptera). Systematic Entomology 43(2): 277-289. https://doi.org/10.1111/syen.12275