This depends on how your instructor chooses to grade the discussion forums. You can generally view your feedback within the gradebook by select the Grades tab:

Followed by looking in the Feedback column that corresponds with the assignment that you are looking for:

If your instructor is using a different grading method, you may have to navigate back to the discussion forum itself and select it. After you open the discussion forum, select View grades:

You will then be able to view your grade and the feedback that your instructor provided.