Which version am I running?

Core version

Go to Administration > Reports > Status report (drupalpath: admin/reports/status) This will list the Drupal version number and much more information about the status of your Drupal installation. 

Or you can use Version Information module, to view version information in a sidebar.

Module / theme version

You can see the versions of your installed modules and themes on the modules and themes administration pages. However, the best overview is on the available updates overview. Go to Administration > Reports > Available updates (drupalpath: admin/reports/updates)

Your installed modules are listed on Administration > Extend (drupalpath: admin/modules) Your installed modules are indicated by a checkmark in the box to the left of the module name. To see the version number of each of your installed modules, click on the description text or disclosure triangle to the right of the module name. This will reveal a listing of information that includes the version number.

Your installed themes are listed on Administration > Appearance (drupalpath: admin/appearance) The version number will be listed in the theme name.