Great thank you Gil for chiming in! in that case the female is much smaller than the male and I assume he is quite old as well. Hopefully I can get her to size before the male passes as they are my only pair of the red form.
Here it would be the shape of the genital operculum. It is round in males and more straight-lined in females.
In this case it is very easy to see (at least for me), but it doesn't mean that it's easy in other species. Some are very tricky to sex externally and the best way to do it is using molts.