Bell has heard about the missing bounty hunters who were after Faadu Naese before them. So when she saw the look on Bianca's face when she opened Faadu's bag, she instantly knew what was inside.
Inside the drawstring bag was the head of Reggin and Orson while their bodies were burned along with the shack.
Bell knew it in just one look by putting the pieces of information together. Call it her super instinct or just her having a good logical and analytical mind.
Faadu Naese, is psychopathic who found sensory fulfillment in killing people without remorse or guilt. He killed his victims out of impulse. He knows what is right or wrong, therefore, he knows that what he was doing or what he did was against the law and he could be punished.
