Is used to measure how trustworthy someone or something is such as when analyzing a narrative mode.?

The term you're looking for is "credibility." In the context of analyzing a narrative mode, credibility assesses the reliability and trustworthiness of the narrator or the information presented. Factors influencing credibility include the narrator's experience, consistency, and the presence of corroborating evidence within the narrative. Evaluating credibility helps readers discern the authenticity and potential biases in the story being told.