It is easy to add sex annotations to hoverflies in the subfamily Syrphinae! Eyes together = male, eyes separated = female.
Here is an article I wrote about it for a UK project, but the same applies everywhere.
Other subfamilies have some exceptions where males have separated eyes, but in Syrphinae you’re safe. Every hoverfly with eyes together is a male though, regardless of the subfamily.
Re: the conversation above about adding them to your own observations while you’re uploading, I also wrote an article about that!