In the test case's right side vertical menu, click to open the comments column. All comments made to a test case will be shown. Click to hide the comments column again. Note that comments posted are specific to the test case version where the comment is made.
Users will need view permissions for test coverage to perform this action. Depending on the settings of a user's project permissions, they may be able to view all test cases or just a subset of them.
Project owners can manage project members project permissions.