If your application is so elaborate to use caching, a debug output in some form is likely to already exist. This allows a practical approach:

1) Reset Cache (or use a fresh instance)

2) The first function call is known to be not cached

3) The second call should be cached. In non-debug mode you can not test this, but results should be equal to 2)

4) Start another instance in debug mode after a cache reset.

5) Call the function two times

6) Compare the debug output, which should contain 'I am (not) cached' type statements.

7) Compare results of all 4 calls.