Determining OS X version from Time Machine backup
Just open the file  

/Volumes/BackupVolumeName/BackupName.ext/ComputerName/latest/VolumeName/System/Library/CoreServices/SystemVersion.plist


to determine the product version ("latest" is a sym-link). 

BackupName.ext, BackupVolumeName, ComputerName and VolumeName are all variable and depend on the type of backup or the names configured/given to your backup volume, Mac and main system volume. BackupName.ext is often Backups.backupdb though.

Here is my SystemVersion.plist (it's the original file and not a file in a  backup) opened in PlistEdit Pro. You may also open it with an arbitrary text editor or Xcode: