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Apart from Small tortoiseshell body size and female egg layers, I thought that the best time to look at different gender wing markings was in the Spring, when two (or more) butterflies were photographed together from a local population.
This picture shows differences in the size of the dark dots on the upper forewings in Spring on over-wintered adults. The female appears to have slightly larger dots on the upper forewing a bit like (but not specifically like) a Large cabbage White.
When this was discussed on someone else show in speech, I was not within "ear shot".
When approaching the female, the male had held it's wings open wider (showing more of the lower forewing). I assumed that a male Small Tortoiseshell might generally fly and position itself, after the female is in position (possibly, generally, behind the female): All contributions from people with expertise, I would welcome here, to help me and others.

I took this shot on the 9th of April last year, my shot shows the two spots on one Small Tort much smaller than on the other. Goldie

Hi Phil, just seen this thread.

I am no expert but I find that both sexes of Small Tortoiseshell can be surprisingly variable, both in size and shape of wing markings and ground colour. I believe that both males and females exhibit a good deal of variation in the dark spots but without a constant difference between the sexes.

I have just posted some photos of courting pairs in the favourites thread at the link below. In all these photos the male is to the rear of the female.

viewtopic.php?f=102&t=10091&p=140490#p140490

Cheers,

Neil.

I would echo Neil's comments, in that both sexes seem to exhibit considerable variation in wing pattern and colour. I don't believe it is possible to sex them with any degree of certainty on the basis of wing markings in terms of size/shape of spots/colour.
Here are two more examples of courting pairs, both also from Lincolnshire.

Thank you Pete Smith & Goldie.
I am looking specifically at the two small dots on the upper forewing (like little dark islands in a sea of "orange").
Pete, both your pictures show that the female (or assumed female) in front had slightly larger dots than the male to the rear. Goldie's picture the same.
I know that this observation will not be 100% accurate, all of the time (constant) but the pictured results are encouraging as a local comparison between sexes.
Another tendency I have noted (certainly not constant), is for the male, when courting the female, to lift it's forewings slightly further away from it's hindwings, leaving more, or all of it's hindwings on display.

Typical courtship pose with male at rear" Ref. Page 200 of : The Butterflies of Sussex (my copy). I read that page and surveyed it this morning.

The male to the rear appeared to have very slightly smaller dark dots (that I referred to above),than the female in front, leaving more of it's hindwings on display (although covering some of the females hindwings).

Kind Regards

The sexes look quite different in my neck of the woods