1) If method B does something, assert whether it happened
2) Create a dummy subclass of the class you are trying to test and have it override method b. Then you can set a variable in your subclass and know if b was called.
Just have a boolean getter and setter methods in your JUnit class, and from MethodB just pass "true" to setter method , so if Method A(Or any Method) called Method B then you will come to know through the value returning by the getter method.
subject: Test for a method containing a call to another method