Unit testing and checking private variable value

I am writing unit tests with C#, NUnit and Rhino Mocks. Here are the relevant parts of a class I am testing:

public class ClassToBeTested
{
    private IList<object> insertItems = new List<object>();

    public bool OnSave(object entity, object id)
    {
        var auditable = entity as IAuditable;
        if (auditable != null) insertItems.Add(entity);

        return false;            
    }
}

I want to test the values in insertItems after a call to OnSave:

[Test]
public void OnSave_Adds_Object_To_InsertItems_Array()
{
     Setup();

     myClassToBeTested.OnSave(auditableObject, null);

     // Check auditableObject has been added to insertItems array            
}

What is the best practice for this? I have considered adding insertItems as a Property with a public get, or injecting a List into ClassToBeTested, but not sure I should be modifying the code for purposes of testing.

I have read many posts on testing private methods and refactoring, but this is such a simple class I wondered what is the best option.