The two fans are controlled (On the V8 anyway) by a pressure switch in the aircon gas line, when the gas pressure rises past a certain level-working it's ba*ls off in very hot ambient temperatures, the fans cut in to cool the gas down and therefore reduce the pressure, either one or two fans will come on depending on what the switch calls for via the HEVAC.
As the Sage from Basingstoke has mentioned, there is also an output via the engine ECU and ECU water temperature sensor (NOT the gauge sensor) that, in the event of the coolant getting excessively hot, the fans will be switched on to assist the engine fan. This function can be checked with diagnostics related to the engine ECU.
My aircon is ice cold, no cheque book symbol and I have not heard my fans come on either. Both pressure switches have been renewed a couple of years ago and the function checked with the Rovacom and, yes they come on when forced.
If the aircon is warm, 98% cause of either no gas, compressor clutch slipping or compressor gone. With aircon on LO, engine running, one pipe to the compressor should be hot and the other freezing cold.