Judging by her reaction {and what else was there to judge by?} he had managed the interview very well.
The quote indicates that the character feels confident in his performance during the interview, interpreting the other person's reaction as a positive affirmation of his efforts. This suggests that the interaction went smoothly, and he believes he successfully communicated his intentions or qualifications.
This self-assurance highlights an important aspect of interpersonal communication: the significance of feedback. The character perceives the other person's response as a measure of his success, reinforcing the idea that responses during conversations can significantly influence one's self-assessment and future interactions.
