Regarding knowing her symbols, I think it's fair that most people wouldn't know much about her at all, but we see based on item descriptions that an onyx stone is associated with her, and Shadowheart wears onyx disks on her armor. So I get the sense that even for the average person, they'd know at least that that is a symbol of Shar the same way an average person in real life knows that a pentagram is associated with Satanism. It's not always associated with that, but it's relatively common knowledge that most people would be somewhat aware of. And when our character knows for a fact she's a cleric, it is not a big leap to make.
As well as the "pentagram thing" is more of a common misconception than an actual common knowledge (check wiki), the onyx thing also not that straight and simple
https://forgottenrealms.fandom.com/wiki/Onyx
