Urriak Uruk
Gaming is fun, and fun is for everyone
...It just now occurs to me that this could be interpreted to mean “it’s the beards that make them hard to tell apart” rather than “it’s the beards that give away the difference” as I interpreted it.
Never mind, ignore me.
Yeah this was the exact scene I was referring to, though I wasn't sure if it was limited to the films.
I did a little more research, and apparently in Appendix A (not sure which book) it is written;
They [Dwarven-women] are in voice and appearance, and in garb if they must go on a journey, so like to the dwarf-men that the eyes and ears of other peoples cannot tell them apart.
And in the War of the Jewels;
...no Man nor Elf has ever seen a beardless Dwarf - unless he were shaven in mockery, and would then be more like to die of shame... For the Naugrim have beards from the beginning of their lives, male and female alike...
I'm not sure exactly where the idea of "beardless dwarf women" actually came from, as Tolkien is of course the base for most fantasy with dwarves. I believe Warhammer Fantasy Battle also has bearded dwarf women (but they're honestly never shown in anything so it matters little either way). Warcraft is I think the biggest property to divert to beardless dwarf women, and a lot of properties have followed that pattern.
Not sure about D&D as there are of course many settings, but the Forgotten Realms wiki confirms that both sexes can grow facial hair, though the majority of females do shave their beards (another departure, as Tolkien dwarven women clearly do not).
Clearly for Exandria, Mercer wanted to call out dwarves to be more like the Tolkien inspiration. It matters little though, as you can always have an individual dwarf shave.