How can I tell if a push comes from merge request or common push in gitlab?

I define rules in.gitlab-ci.yml to filter different events. such as

merge_request_check_job:
  script:
    echo "merge request event"
  rules:
    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
      when: always
      allow_failure: false

merge_request_check_job:
  script:
    echo "push event"
  rules:
    - if: '$CI_PIPELINE_SOURCE == "push"'
      when: always
      allow_failure: false

In GitLab CI, a push event can be triggered either by a normal code submission or by a push triggered by clicking the Merge button under the Merge Request menu. How can i tell the difference between these two.