It's a bit odd but I can't find a way to do this other than using the Reports section, which is obviously not ideal but it does at least enable you to identify the test cases that are in a folder (by filtering on the folder you want, "Folder A" in your example). Ideally the Filter option in the test case view should enable you to add criteria but it just defaults to ALL.
I've added my vote for this feature to be added to the Cloud version and you can do the same here: TestCase Explorer: Improve Filtering for | Zephyr Scale Cloud Ideas (aha.io) (you'll need to register as a user on the board if you're not already).
