In reference to my When Do We Need Detailed Test Cases? post, Roshni Prince asked:

“when we run multiple tests in our head… [without using detailed test cases] …how can we be really sure that we tested everything on the product by the end of the test cycle?”

Nice question, Roshni.  I have two answers.  The first takes your question literally.

• …We can’t.  We’ll never test everything by the end of the test cycle.  Heck, we’ll never test everything in an open-ended test cycle.  But who cares?  That’s not our goal.
• Now I’ll answer what I think you are really asking, which is “without detailed test cases, how can we be sure of our test coverage?”.  We can’t be sure, but IMO, we can get close enough using one or more of the following approaches:
1. Write “test ideas” (AKA test case fragments).  These should be less than the size of a Tweet.  These are faster than detailed test cases to write/read/execute and more flexible.
2. Use Code Coverage software to visually analyze test coverage.
3. Build a test matrix using Excel or another table.
4. Use a mind map to write test ideas.  Attach it to your specs for an artifact.
5. Use a Session Based Test Management tool like Rapid Reporter to record test notes as you test.
6. Use a natural method of documenting test coverage.  By “natural” we mean, something that will not add extra administrative work.  Regulatory compliance expert and tester, Griffin Jones, has used audio and/or video recordings of test sessions to pass rigorous audits.  He burns these to DVD and has rock solid coverage information without the need for detailed test cases.  Another approach is to use keystroke capture software.
7. Finally, my favorite when circumstances allow; just remember!  That’s right, just use your brain to remember what you tested.  Brains rock!  Brains are so underrated by our profession.  This approach may help you shine when people are more interested in getting test results quickly and you only need to answer questions about what you tested in the immediate future…like today!  IMO, the more you enjoy your work as a tester, the more you practice testing, the more you describe your tests to others, the better you’ll recall test coverage from your brain.  And brains record way more than any detailed test cases could ever hope to.