Take it to a jeweller and they will tell you free of charge. They can tell as soon as they take the back off. There are resources in the main link to help you, but removing the back and having an expert look is the only way. China is making lots, but they have a cheap Chinese watch movement inside them. It can be quite difficult to tell fake Cartier watches from genuine Cartier watches, but there are a few guidelines you can follow: Inscription The simplest way to tell a fake Cartier watch is to remove the caseback.
The question: The Rolex has the papers, no concerns there, but the Cartier does not. It's in good shape, but of course, before I complete the purchase I'd certainly like to know if it's fake. I trust the seller, I do not think the seller would knowingly try to deceive me. I don't have the watch in hand, but I have the serial number.