The state of private fields is irrelevant to the correct functioning of the class. You shouldn't be testing private variables.
Test what the class does, not how it does it.
If you want to test that the class sets its internal state correctly, so it behaves in a particular way after that, then call the methods that set the state and test that the behaviour after that is correct.
It doesn't matter if it takes 1000's of tests to ensure that - they are only unit tests, so they'll be fast.
Future coders are at liberty to change the internal workings of a class, as long as the class' contract is upheld.