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Well, there are 2 answers

for Tachinids: look at the abdominal tip. 100 % sure
95 % sure: females have 2 pair of additional bristles on the vertex near the eyes, pointed FORWARD (the proclinate orbitals). Males don't (with 5 % exceptions)

For Gymnosoma:
Males have bicolorous thorax: front golden yellow, back black.
Females have completely black thorax.

Simple comme bonjour (once you know it)


Theo