uhhh, without knowing what car you have, I'll just explain a bit about the catalytic converter and o2 sensors on your car.
if the car is newer than '94, then it likely will have 2 o2 sensors per catalytic converter. The first one is used by the computer to help control the fuel ratio - the main purpose being emissions, then mileage. The second o2 is to monitor the effectiveness of the catalytic converter. If it has a problem reducing/oxidizing the engines emissions, then a code will be set from the signal of that second o2. The same goes for if the sensor itself has a problem...a code may be set.
If you have factory dual exhaust then you may have 2 cats and 4 o2 sensors. The reason there may be some confusion over how many cats you have may be due to the fact that there are two main parts to each cat itself and is usually internally divided. One section is designed to do a 'reduction' chemical reaction (primarily for Nox emissions) and the other section will do an oxidation reaction (for changing carbon monoxide and hydrocarbon emissions to make them less harmful).
Catalytic converters will almost always be as close as possible to the engine. The reason for this is that they will heat up to their necessary operating temperature quicker and stay at that temp better in all driving conditions.
Other components that may seem similar to cats are : mufflers, and resonators..However if you can be virtually sure that the cat is the device right inbetween the two o2 sensors on one exhaust pipe. One o2 in front of the cat, and one just downstream of the cat.
hope this helps....
boa