ansible/test/integration/targets/blockinfile/tasks/diff.yml

- name: Create a test file
  copy:
    content: diff test
    dest: "{{ remote_tmp_dir_test }}/diff.txt"

- name: Add block to file with diff
  blockinfile:
    path: "{{ remote_tmp_dir_test }}/diff.txt"
    block: |
      line 1
      line 2      
  register: difftest
  diff: yes

- name: Ensure diff was shown
  assert:
    that:
      - difftest.diff | length > 0