The Apple menu has an entry that will let you know what version of the OS you are running.
Click on your desktop to make the Finder active, then click on the Apple menu found in the upper left hand corner of the screen. The first item in the menu will say one of the following; About This Macintosh, About This Computer or About This Mac. Click on this menu and a window will open showing the OS version.
If you know you are running Mac OS X or later: There is no need to click on your desktop to activate the finder, the Apple menu is always active. The ‘About’ window will show you the version and build number of the OS you are running.