The job of the condenser is to reduce the temperature of the refrigerant. The process of compressing the gas by the compressor generates not only high pressure but also heat.
Heat is dissipated by radiation from the condenser, very much like in the case of a cooling radiator. It is located usually in front of the vehicle radiator so that the cold air coming through the vehicle grill goes through the condenser first.
Condensers must be in an adequate airflow, and the engine cooling fan is often incorporated as a function of cooling the condenser.
The fan is usually mounted on the engine side so it sucks air through the condenser, rather than blowing it under pressure. A depression (sucking the air) is cooler than a pressure (blowing the air).
A separate cooling fan is often used for air conditioning.
How to tell it is working?
When refrigerant is flowing through the condenser not only will the fans usually be running, but the condenser will be hot to the touch – it is high pressure refrigerant running through still so it will contain heat.
The refrigerant will turn from a gas to a liquid in the condenser. As it is cooled down a small amount, it condenses.