Unit test helper methods?
I have classes which previously had massive methods so I subdivided the work of this method into 'helper' methods.
These helper methods are declared private to enforce encapsulation - however I want to unit test the big public methods. Is it good to unit test the helper methods too as if one of them fail the public method that calls it will also fail, and this way we can identify why it failed?
Also in order to test these using a mock object I would need to change their visibility from private to protected, is this desirable?
Answer
This smells like you have the wrong problem. What you've described is akin to creating a sub-"unit test", which leads me to believe that your unit tests are really testing a unit after all.
Which is not a criticism of what you are trying to do: going from "where we are today" to "somewhere else that's measurably better" is a winning move. However, it is a suggestion that you step back a bit to evaluate where you are - understanding how your current situation differs from some Platonic ideal could help to show new possibilities.
There are plenty of suggestions here about scoping your helper methods. Another possibility would be to review the implementation to determine if there are helper classes lurking in your current implementation. Creating a new class and a suite of tests to exercise it is always acceptable.
Note that this approach insulates you from the refactoring: you can change the implementation without changing your test suite (because the unit tests for the helper object continue to pass even when the helper object is no longer part of your production implementation), and you get a clean packaging of the implementation and the tests for it (usecase: you decide that bozo-sort is the wrong implementation, and should no longer be used. If the bozo-sort implementation is isolated, then you simply remove it and its tests. But when the tests of the bozo-sort implementation are tangled with all of the other tests, there's more thinking involved).
It may also help to review why you have unit tests for your code. If one of the reasons is "to make refactoring safe", then you don't want to be writing tests that lock you into an implementation.