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:
The simplest way to tell a fake Cartier watch is to remove the caseback. A genuine Cartier watch will have "Cartier" inscribed on the movement.
Like most luxury accessories, genuine Cartier watches are quite heavy, especially when compared to the lightweight fakes. This solid weight is due to the watch's quality materials.
A genuine Cartier watch has scratch-proof glass, which won't be found on the fakes. This is something that's fairly easy to test. The sapphire-quality glass of a genuine Cartier is also non-smear, so if you drop a few beads of water on it, they will roll off. On fake Cartiers it will smear.
The winder of a genuine Cartier watch features a polished gemstone, called a cabochon stone. If there's no stone, it's a fake Cartier watch. The gem should be securely fastened, not glued on.
This should appear on the back or side of a genuine Cartier. The number will be engraved as a number of fake Cartiers feature a serial number that's barely etched onto the watch.
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