You can easily distinguish male and female flies of the family Platypezidae* by their eyes. In males, the eyes touch each other in the middle, whereas in females, they don’t. Also, in many species the males are almost completely black, where as the females are more ornate.
I had done a lot of annotations at some point and got the graphs on the species page very nice, but I haven’t managed to keep up with them this year, so there are a lot of un-annotated ones again. If this sounds like something, you’d be interested in, I’d appreciate the help! :)
*same goes for other flies as well, but not all of them, I believe