I read other eBay guides on how to identify authentic Gillette blades, but could never find any of the markings I was told to look for, even on new blades I bought at full retail in my local Rite Aid drug store.  After getting burnt purchasing fake Mach 3 Power blades on eBay, I finally emailed Gillette to get the company's official position on how to identify fake blades.  Here is the answer I received:

Thank you for contacting Gillette.  There is no way to determine if the product is counterfeit unless it is examined by the quality team.  Cartridges that are intended to be sold in the United States will have English and French languages on the packaging.

Thank you.

Amanda
Gillette Team