A good way to check is to get a pic of the end links in places where they aren't 'on display' and the inside of the main part of the clasp. These are areas that a copy would be less accurate. Also, you can check under the end links to see the finish on the case, again not always finished well on a fake.
Check how the bezel rotates, it should feel solid and well made. Fakes will often have 'play' in them.