On the day an assignment is due, I ask the students to review their work and answer a series of questions. Identify one particular problem that you encountered in writing this paper. The self-critique is helpful to the students because it provides a beginning toward evaluating their own works objectively. This self-critique exercise is also useful to me. Finally, this technique gives the student a chance to alert me to problems that otherwise might have remained hidden.
Some students tend to be more open when writing to rather than speaking with the instructor.