How to trace the user module

From time to time I have the situation, that the user module ties to change somthing, but I have no idea what. And when I proceed to run Ansible some change gets done, but I have no idea what might be different after the change. The user module does not seem to support the diff option. How can I analyse what the user module tries to change?

The docs at http://docs.ansible.com/ansible/latest/user_module.html don’t mention it, but the user module does

return some info, so you can register its results and display it with debug.

Example playbook: