If you need to find the OS version you are using and cannot use the usual methods (for example, booting from recovery partition or you only have terminal available, such as when in an SSH session)
Open these files (one for standard OS, one for server) and look at the settings labelled "ProductBuildVersion" and "ProductVersion".
In Terminal you can use the "cat" utility to display the contents, for example, type: "cat /System/Library/CoreServices/SystemVersion.plist" (without the quotes).