unfortunately fakes these days can look REALLY good. As someone who likes a few key designer pieces (cartier love is one of them!) here is what I do when I purchase something designer pre owned, be it jewelry or purse - gleaned from my years when i used to troll the purse forum more than PS
Go to the store- in this case, cartier. Tell the you want to drop off your ring for a buffing and re-etching as it has gotten worn from wear. The SA will take it from you and inspect it quickly. Most will know within seconds if it is a fake. If it is, they will give it back and say they cannot service the piece because it is not authentic. To take it one step further if you want to be SURE sure, send it off for servicing. It used to be $140-$175 at Cartier depending on the piece, and it goes to NYC to be all spiffyd up. For sure they will catch it if it is not authentic.
I have done this with preowned cartier, tiffany, and van cleef pieces. ONE time something I bought was a fake, and I was of course flustered even though i knew deep down that could always happen. I blamed it on an ex
For what it's worth, I have heard good things about True Facet. hopefully it's legit!