Assuming we all know how to check the drupal version if your site is online and you know the admin password… (Administer > Logs > Status Report)
But what do you do when you only got a zipped backup of your drupal site or the site is offline or you don’t have the admin password for the it?
Look inside the CHANGELOG.txt file in your drupal root dir. There may be several strings that look like Drupal versions i.e. something like “Drupal 7.37” – find the latest one, that’s your current drupal version.