A. More than four antennal flagellomeres:

The antenna has more segments in more primitive flies. Therevids have three main segments and an apical style comprising two of three segments. If your specimen has more than this number then you have a fly from any one of a large number of more basal families in the "Nematocera" or of the infraorders Tabanomorpha, Stratiomyiomorpha or Xylophagomorpha. Some families in these groups which could be confused with therevids include Stratiomyidae, Mycetophilidae, Tabanidae, Rhagionidae, Xylophagidae and Xylomyidae.

B. Empodium pulvilliform:

The middle article of the foot, the empodium, is pulvilliform (i.e. broad) in the aforementioned groups of lower flies (some "Nematocera" and infraorders Tabanomorpha, Stratiomyiomorpha or Xylophagomorpha) and also in Nemestrinidae and Acroceridae. Therevidae, along with all other higher flies have a bristleform (narrow and 'bristle'-like) empodium.