How to unit test the std::bind function using gtest?

I am trying to write unittest cases for some of the cpp files in my project.

The scenario here is: I have a cpp file with only one public method defined and in turn which calls private methods.

Here the private methods are called in the public method as a callback method. How do I test the private methods here. I will be doing the mocking for Callback pointer and I am not sure how to call the private method.

Please give me some suggestions how to call the private methods in this scenario without changing the source code.