Ok, so you have a random disk lying around and you plug it in and it looks like it has Mac OS X installed on it. How do you tell what version it is without booting into it?
First, launch terminal and cd into the root of the disk (“/Volumes/whatever“). Then run this command:
sed -e 's/.*\(10\.[[:digit:]]*\.[[:digit:]]*\).*/\1/' \ -e '/^10/q' -e d System/Library/CoreServices/SystemVersion.plist
Magic! 