How do I find out what version of Redmine I have?

To find your Redmine version, the easiest way is through the web interface: log in as an administrator, go to Administration → Information, and you’ll see the version listed. You can also visit http://YOUR-SERVER-NAME/admin/info directly. If you have server access, check the $INSTALL_DIR/doc/CHANGELOG file or run Redmine::VERSION::STRING in the Rails console. Without admin or server access, it's hard to tell precisely, though some clues may be visible in the footer or features.