My last coach was a 1995 P30 with a 454 and the service manual had a procedure for checking the fan clutch. They said to measure the temp of the AIR coming out of the radiator. I used a metal meat thermometer. I had to drill a small hole in the plastic shroud around the fan so I could get the thermometer between the fan and the radiator.
They said to block the radiator with cardboard and run the engine at about 2000 RPM to get the engine to heat up.
The fan will spin before it's fully engaged, but it's obvious when it engages because of the noise.
It should engage at an air temp between 165'F and 195'F. If it doesn't it should be replaced.
Clay WA5NMR - Ex Snowbird - 1 year, Ex Full timer for 11 years - 2004 Winnebago Sightseer 35N Workhorse chassis. Honda Accord toad.