Okay, I have purchased a great number of Cartier glasses on Ebay and I have learned how to spot real from fake. 
1.  Always check the serial number on the glasses Cartier has a seven digit serial number that usually starts with a three(well from the authentic pairs I've seen it has always started with a three)
2. Once you see the serial number call  Cartier at 18004477405 and read of the serial number and make sure the glasses are as described. 
3.  Make sure the glasses are at a decent price usually if you see a pair of Cartier glasses under $200 then they are not real, but some are not real that cost that much
4.  All  the newer authentic Cartier glasses come with a black hard case, usually the fakes have a soft leather pouch to carry the glasses in instead of a hard case, so look for the black hard case or a brown leather hard case for the older ones.
5.  Look at the screws on the glasses.  Cartier has little metal round screws not plastic
6.  Look for an authenticy booklet but this does not always mean much.  I have seen plenty of fake Cartier glasses with all the paper work so don't be fooled.
7.  If you see a big serial number on the glasses then they are fake.  Cartier always has the serial finely engraved somewhere on the glasses very small and fine and its usually hard to see. 
these are all the tips I have for spotting the fakes and there is only one store I know of on Ebay with real Cartier Glasses and that is the store entitled cartier glasses.