You can test all four radiator fan relays (2 high and 2 low) by jumpering the relays (if I recall correctly it's pins 87 & 30), or with a Durametric. You can find instructions and the relay location by doing a search. The engine compartment fan is triggered by the ECU based on several factors, you can also find this info by searching.
http://986forum.com/forums/diy-project-guides/60239-engine-compartment-fan-manual-switch.html