code: public boolean definesSerializableFields() {
          if (!isSerializable() || isExternalizable()) {
              return false;
          } else {
              if (serializedForm == null) {
                  serializedForm = new SerializedForm(env, tsym, this);
              }
              //### Clone this?
              return serializedForm.definesSerializableFields();
          }
      }
docs: Return true if Serializable fields are explicitly defined with
  the special class member <code>serialPersistentFields</code>.
  
  @see #serializableFields()
  @see SerialFieldTagImpl