How can I identify what version of Drush is installed on a given system?
Assuming you have Drush installed and working properly, just type
drush status in your command prompt. It will give you drush version as well as Drupal version info. If you have aliases set up for remote (or local) sites, you can run
drush @alias status and it will give you the status info for that remote system.
If your Drush installation is not working and you have an older version of drush, then neither
drush status nor
drush --version will help. In that case go to the directory where drush is installed (~/.drush/drush on my system) and look at the drush.info file. There you will see a line that says
version=3.0 (or whatever)
You can check Drush version with "drush status" command inside your application directory or outside your application directory, the difference will be when you are inside application directory you will get more information,
Check below links for better understanding on Drush commands,