There is an example which you have probably seen at https://github.com/ansible/ansible-examples/blob/master/language_features/prompts.yml which uses vars_prompt which may be helpful as a reference.

I usually use the debug module if I am unsure if a variable is reading in the proper values. You’ve probably used that module before, but the documentation for that is at https://docs.ansible.com/ansible/latest/modules/debug_module.html.

Based on the above examples, one test that you could use might look like this:

---
- hosts: localhost
  vars_prompt:
    - name: "some_password"
      prompt: "Enter password"

  tasks:
    - debug:
        msg: "{{ some_password }}"