Finding the version of OS X on a live target system is straight-forward.  You simply select the Apple icon located in the upper left corner then select “About this Mac.”  Here you can find all sorts of system information to include the version of OS X.

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On a dead box exam you must check the SystemVersion.plist located at:

DISK > System >Library > CoreServices > SystemVersion.plist

If you have Xcode installed, which is a free add on to your Mac available in the iTunes store, you may use it to view the plist.  You may also highlight the plist and tap the spacebar to view it in quicklook.  There are also third party plist editors available, such as Plist Editor Pro.  Installing Xcode is likely your best option.

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