How about this: She makes it clear that she's a cleric, and is wearing a diadem with a single symbolic decoration in the centre of her forehead - it's not reasonable to argue that it could be dismissed as not symbolic, which means that literally anyone contemplating it (short of folks with int and wis of 6 apiece) will identify it as a deliberate symbol that this cleric is wearing. Then we're at the question of 'what is it a symbol of, and who would recognise that', and once again, we're at the truth that Selune and Shar are known as facts of the world in this region - this exact region of the world has been their traditional stomping ground since Toril came into existence, and not long ago their literal divine avatars had a war in a nearby city and the surrounding lands.
As others mentioned, that war was classified as a realms-shaking event, which in the lore as written is intended to mean that it's an established and known event that literally everyone knows about, to some extent - I.e. Even people who live in Shou know this war between selune and shar occurred on the sword coast in faerun.
The idea that someone who lives on the sword coast would not look at her forehead, and see a symbol of shar, is quite legitimately a ludicrous non-starter in this context; supposing anything else is bringing our own real-world sensibilities into the matter. In This setting, here and now, there is a near-zero percent chance that an average muggins who lives in this region of the world wouldn't see it for what it is. At best, they might assume that she's posing with the intent of being edgy or scaring people off... but they'd recommend she pick a less objectionable symbol, since this one will get her lynched at worst and chased out of town at best.
