The easiest way to spot a fake Cartier watch is to look for the inscription and serial number. Inside the back case, a real Cartier has "Cartier" inscribed on the movement. A real Cartier also has the serial number engraved; on a fake, it is barely etched.
There are a few other ways to distinguish between a real Cartier and a fake. A real Cartier watch is heavier than a fake one. This is due to the quality of the metals in the watch. The glass on a real Cartier is scratch resistant and non-smear. Also, water rolls off the real watch. Lastly, a genuine Cartier has a polished gemstone that is fastened in and not glued.