You'll hear a distinctive roar when it cuts in. The first time mine cut in, I thought an aircraft was flying overhead. The service manual for the 1998 12 valve describes the testing procedure at section 7-21. First it states that the fan is defective if the blades revolve more than 5 turns when spun by hand. The next test involves inserting a dial thermometer with an 8 inch stem through a drilled hole in the radiator shroud, putting a sheet of plastic in front of the radiator, and idling at 2400 rpm. Within 10 minutes the fan should engage and start roaring at 160 to 179 degrees F, and cut out when the temp. drops to 135 to 175.
Nick.