Prior research has established that learning by teaching depends upon peer tutors' engagement in knowledge-building, in which tutors integrate their knowledge and generate new knowledge through reasoning. However, many tutors adopt a "knowledge-telling bias" defined by shallow summarizing of source materials and didactic lectures. Knowledge-telling contributes little to learning with deeper understanding. In this paper, we consider the "self-monitoring hypothesis", which states that the knowledge-telling bias may arise due to tutors' limited or inadequate evaluation of their own knowledge and understanding of the material. Tutors who fail to self-monitor may remain unaware of knowledge gaps or other confusions that could be repaired via knowledge-building. To test this hypothesis, sixty undergraduates were recruited to study and then teach a peer about a scientific topic. Data included tests of recall and comprehension, as well as extensive analyses of the explanations, questions, and self-monitoring that occurred during tutoring. Results show that tutors' comprehension-monitoring and domain knowledge, along with pupils' questions, were significant predictors of knowledge-building, which was in turn predictive of deeper understanding of the material. Moreover, tutorial interactions and questions appeared to naturally promote tutors' self-monitoring. However, despite frequent comprehension-monitoring, many tutors still displayed a strong knowledge-telling bias. Thus, peer tutors appeared to experience more difficulty with self-regulatory aspects of knowledge-building (i.e., responding appropriately to perceived knowledge gaps and confusions) than with self-monitoring. Implications and alternative hypotheses for future research are discussed.