In the absence of detailed requirements or a specification, it might seem impossible to figure out what the expected behavior of the system should be. However, you have one of the best resources available to determine expectations: the software itself. You also have access to other resources. Depending on the context, you may be able to gather information about other, similar software packages. Or your software may be governed by publicly available standards.
Users of your software have every reason to believe that it will be self-consistent, so the software itself can help you evaluate the correctness of the behavior you observe.
While exploring an application, you find that you can ...