This forum post (https://forum.inaturalist.org/t/sexing-lepidoptera-conversation-and-references/6702) came up in a recent google search of mine but I missed my opportunity to respond.
I’ve found literature thats reported the use of the frenulum as a useful characteristic for externally sexing many moths species. I’m wondering if this has become an outdated method otherwise it is a very nice trick.
The frenulum is a hair like process that extends from the basal portion of the hindwing. As far as in know in many moth species this structure is sexually dimorphic where males have only one bristle and females have more than one (usually 2 to 3). There are definitely species and families that deviate from this trend so if you are doing some serious work you’ll definitely have to double check that this is diagnostic.
First saw this in Zimmerman’s 1978 hawaiian lepidoptera but also found it here.