If you have a specific hypothesis about how your cases will differ, then you can collect Structured Observations to assess that hypothesis. I prefer to say "assess" (or "evaluate") rather "test" for this situation because the latter term implies statistical testing, which is probably not possible.

For comparative case analysis, I recommend Yin, Case study research: Design and methods.

If you are using your observations in a structured way to test pre-existing ways of understanding your case, then yes.