To determine your website's installed Drupal version, go to
http://[site_URL]/admin/reports/status. The Status report page opens, and the Drupal version is to the right of the Drupal variable in the table of your website's parameters.
Here is an example of a Drupal 8 website:
Here is an example of a Drupal 7 website:
Here is an example of a Drupal 6 website: