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Simply go to this URL https://selfsolve.apple.com/GetWarranty.do and type in your serial number and select your country. This is Apple's Online Service Assistant where you can find details about your Mac's warranty, and if your machine is with Apple for servicing, you can track it here as well.
If you do not know how to find your serial number - it’s easy.
Simply click on the Apple logo on the upper left hand corner of your screen, and select “About This Mac”. A small window will appear and click on “More Info...”. After doing so, a new window with more details will display and at the bottom area of the list you will find your serial number there. This window is the System Profiler, where you can see the technical specifications of various items that are built into your Mac.

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You can copy your serial number to your clipboard from here, and paste it into the form in the Online Service Assistant form. After entering these two fields, you can click continue and find your warranty details.

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It will be indicated in the resulting page when your warranty expires for the type of machine you have. It will be indicated here if your machine is out of warranty or not.