Bugs can be linked to a test case when that test case is executed (i.e. during test sessions in the project's Build tab). To view the bugs linked to a test case version, navigate to the specific version of the test case. Click on the icon on the vertical menu on the left side of the test case details window. It will expand to show the list of linked bugs, if any. If there are bugs linked to a test case version, the number will show under the icon.
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 and bugs or just a subset of both.