Drupal - How to check Drush version?

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.

drush --version works in Drush 4 too.

You can also use drush st as a shortcut (alias) for drush status