The ansible_distribution_major_version magic variable will return the major version of a managed node (e.g. the target system). Refer to run tasks on the control node if you want to run this on your control node.

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

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

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_distribution' 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"}

Running this playbook should return the following.

TASK [debug]
ok: [server1.example.com] => {
    "msg: "7"
}