I'm not aware of an official lore answer on any of this, however:
The Eliksni ('Fallen,' to most of humanity) are insectoid/arachidlike in biology (https://www.destinypedia.com/Fallen#Biology).
Much of their physiology seems to be more spiderlike, in particular, but this is already into conjecture as Earth spiders and the Eliksni have no common biological origins. That said, if the spider/arthropod model holds then we can expect Eliksni sexual dimorphism to express itself in 1) differential coloration, 2) size of certain bodyparts (mandible, pedipalps, etc), and 3) the presence of sex-specific genitalia.
#1 is no good, because (like Spiders) the Eliksni exhibit a wide range of coloration, and the consumption of ether intervenes in whatever natural growth cycles they might have - plus the Eliksni we see tend to all be wearing armor anyway, or otherwise decorating themselves according to their House.
#2 is no good, because again, ether intervenes in the natural growth cycle.
And #3 is no good, because Destiny would have to be an ENTIRELY DIFFERENT SORT OF GAME for us to be able to examine them.
As to how the wiki knows the gender? The writers assign the gender (or its assumed/inferred from casting choices) so they can tell male from female because they're deciding it.
Even then, not all cases are decided - case in point: https://www.destinypedia.com/Weniks,_Salvation_Elite Whose pronouns are They/Them and no gender is given in the sidebar. It doesn't seem to be important to the writers.