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Validation
The only ways to know if it is real or fake is to buy from an AD, a trusted dealer, or a reputable third party. You could take the watch to an AD and have him look at it.
The warranty travels with the watch. It's best to have the Certificate of Origin or the original sales slip if you can get it. Patek doesn't care who owns the watch provided it was sold through legitimate channels.
If you own the watch you should be able to register for the magazine by submitting the movement number (see web site).