The simplest answer I know of is that I know whatever is evident or obvious to me. If there is never anything evident or obvious to you, then you have no capacity to know. The third can be dealt with if one recognises that in order for anything to be evident or obvious one has to attend to it or take an interest in it in some way. Inter alia, one might be said to study, in order to know anything. Knowledge and methodology can be regarded as the foundation of any discipline.
Epistemology is a part of the foundations of any discipline that seeks knowledge.
Specifically epistemology is the study and development of methods and grounds of knowledge. If epistemology is to be a serious study the answer will be 'anyone' or at least 'anyone who does know'. Epistemology may be a distinct discipline whose subject is those who know but is in any case an integral part of any discipline which seeks knowledge. Where epistemology is a part of a discipline then the methodology will be that of the discipline of which it is a part, but epistemology is of broader scope than any specific study and the specific methods which comprise it. For instance whilst mathematics might be fairly characterised as the study and development of specialised methods of knowledge, the epistemology of mathematics would include the study of learning and teaching mathematics and its application in any walk of life and thus encroach on any field where mathematics is employed.