Unit test private inner class

I have a Class A which has an internal Cache represented by a Class B. this inner class is private since the cache need not be visible to external consumers and is only to assist the outer class A. I am using Jmock and Java

public class A {
    ...
    private class B {
    ... 
       public void testMethod() {
          //The method I want to unit test
          ...
       }
    }
}

This is shown above. I am not sure how to unit test the testMethod() which is from private inner class B (as class B is not visible to the outer world).

Please advise!

Thanks!