The ansible_play_hosts magic variable can be used to output the managed hosts the play is being run against. 

For example, let's say you have the following playbook. 

---
- hosts: all
  tasks:
    - debug:
        msg: "{{ ansible_play_hosts }}"

If you get the following output, the gather_facts module may be set to false. gather_facts must be set to true, or not defined at all in your playbook, as gather_facts is true by default.

TASK [debug]
fatal: [server1.example.com]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'ansible_play_hosts' is undefined\n\nThe error appears to be in 'testing.yml': line 8, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n  tasks:\n    - debug:\n      ^ here\n"}

Something like this should be returned.

TASK [debug]
ok: [server1.example.com] => {
    "msg": [
        "server1.example.com", 
        "server2.example.com",
        "server3.example.com"
    ]
}

The same could be done with the ansible_play_batch or ansible_play_hosts_all magic variables, which should return the same exact output.