There are two events that trigger the auxillary fans. The first is the refrigerant pressure switch, which turn on the first stage (low speed) when refrigerant pressure passes a certain pressure threshold. The second trigger is the ECT sensor, which turns on the fans at stage 2 (high speed) at a pre-set engine coolant temperature.
To test if your pressure switch is working, pull out the plug at the top of the freon receiver. This should turn on the fans in low.
To test if the ECT sensor is working, pull out the connector on top of the thermostat housing. This will trigger High speed.
This works for the W202. On some models you might need to jumper together the terminals on the connector.
If no go, check out the fuse supplying power to the two aux fan relays. The fuses are identified on the label beneath the fusebox cover.
If all else fails, see if the fans themselves are functional by applying 12 VDC directly to their terminal. Locate the dropping resistor which is connected to the fans and apply 12VDC. Watch your fingers!
Hope this helps