Even in the game, it is clear, Shadowheart makes it clear herself, people don't like Sharrans. She's nervous you'll immediately boot her from the party because she's a Sharran. It's made totally clear, if you trigger the right dialogues and pick up the right books that Shar is a well known bad goddess.
And the mural. That's the biggest indicator. You don't need a roll when you look at the mural. The game just tells you, "The bad guys in the mural have the symbol of Shar.".
So you recognize it there, but for some reason, nowhere else.
