Engine off. Grab one fan blade, push it forward-backward (toward radiator then towards engine.) If you feel looseness in the fan clutch bearings, you need another fan clutch.
LOOK at the front of the clutch. OEM and OEM-style clutches will have some kind of "spring", usually a coil but sometimes a leaf spring. If you see more than tiny traces of oil which will then attract dirt, you need a new clutch.
If there is NO spring, and NO electrical connector/wire harness, you've got a NON-THERMAL (Centrifugal) PIECE OF CRAP FAN CLUTCH who's only purpose is to pry money out of people too cheap to buy the CORRECT parts when the original failed. Non-Thermal fan clutches are JUNK. You need a new THERMAL fan clutch.
Bearings good, no leakage, has a spring of some sort on the front? Start engine, block the throttle open to "fast idle", 1500 RPM to perhaps 2000 rpm. When the clutch is DIS-engaged, you'll think there's a lot of air movement, and you'll think there's a lot of fan noise.
Cover the front of the radiator with a couple of paper towels or shop rags, near the "hot" tank and near the "cold" tank, but NOT IN THE MIDDLE. Leave the middle half uncovered, with 1/4 of the left side and 1/4 of the right side blocked with paper towel, cardboard, shop rag...whatever.
Let the engine warm up. At some point, if you've covered enough of the left and right sides of the rad core, BUT LEFT THE MIDDLE OPEN, the engine will get hot enough to engage the fan clutch. (Keep an eye on the temp gauge, don't let it go over 230-ish degrees F. (It should not take that much engine heat, but I don't want you to boil the coolant if the clutch is defective.)
HO-LEE CRAP, the airflow will SUDDENLY AND DRAMATICALLY increase, the noise will SUDDENLY AND DRAMATICALLY increase, Dorothy and Toto will fly past the car, and YOU'LL KNOW that the clutch engaged.
If the temp climbs and the airflow/noise doesn't suddenly and dramatically increase...you need a new clutch, OR YOU NEED A NEW RADIATOR.
The fan clutch engages based on the temperature of the airflow coming off the radiator. If the radiator can't transfer heat to the airflow, the fan clutch won't get hot enough to engage. LOTS of fan clutches get returned under "warranty" because they're "faulty" when the truth is the radiator is shot--can't heat the air. Either the coolant tubes are full of scale, or the air fins have corroded away from the coolant tubes. Either way, the heat isn't transferring to the air, and the clutch doesn't get hot but the engine does.
DO NOT stand in-line with the spinning fan. Those blades will--occasionally--pop off and fly through the hood, or into the pavement, or through you. This will be...unpleasant.
MANY but certainly not all GMT400 fans do not have enough blades or blade pitch to make proper use of a "heavy duty" or "severe duty" fan clutch. Those are NOT an upgrade for MOST GMT400 fans, especially 1500s and not-Diesels. The "heavy duty" and "severe duty" fan clutches will not properly disengage when the engine is cool/warm; you'll lose MPG and gain excess fan noise. They will cool better even when you don't need extra cooling...because they're never properly disengaged.
