Social epistemology

Social epistemology is essentially the study of what significant contributions are made by some social mechanisms or other to our gaining of knowledge and justified belief. So one central topic in social epistemology is "testimony," construed broadly--i.e., the habit we have of learning stuff from other people. One central question in social epistemology is: assuming that we are very often justified in believing something based on the testimony of other people, where does this justification come from, and in particular, does it necessarily come from some observations we have of those other people's reliability?