John is right on with his advice. The coolant should be replaced every 3 years, but more importantly I should have mentioned the thermostat. Wouldn't hurt to replace it.
I've never replaced very many fan clutches, seem more likely to get broken in an accident than to just "fail". But the fan is seperate of the clutch and can be installed backwards, that is, blowing air from the engine to the radiator rather than sucking air through the radiator from the grille. Just run the engine and put your hand by the front of the air cleaner, should be easy to tell if it's working correctly.
You can pull the couple of clips that hold the radiator in place and carefully tilt the radiator towards the engine, and using a flashlight you can look between the radiator and A/C condensor to check for external blockage.
400E: This checks everything for the engine control of the fans EXCEPT the sensor itself. The sensor can be checked using a chart and a ohmeter.
The A/C low speed fan can be checked by shorting together the high pressure switch, except for models using a dual fuction pressure switch, I can't swear that it will work on those models. The sensor for the A/C is checked by monitoring pressures of the A/C system while monitoring the closing of the switch with an ohmeter.